feat: add support for policy tags

[shollyman]

State: unit testing done
Pending: system tests

Fixes: #74

[shollyman]

@plamut I'm not understanding the coverage failures from the most recent run, which don't appear to be related to this PR. Any insights?

Running session cover
Creating virtual environment (virtualenv) using python3.8 in .nox/cover
pip install coverage pytest-cov
coverage report --show-missing --fail-under=100
Name                                                Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------------------
google/cloud/bigquery/__init__.py                      57      0      0      0   100%
google/cloud/bigquery/_helpers.py                     237      0    120      0   100%
google/cloud/bigquery/_http.py                         13      0      0      0   100%
google/cloud/bigquery/_pandas_helpers.py              262      0    125      0   100%
google/cloud/bigquery/client.py                       642      2    272      0    99%   22-23
google/cloud/bigquery/dataset.py                      252      0     61      0   100%
google/cloud/bigquery/dbapi/__init__.py                30      0      0      0   100%
google/cloud/bigquery/dbapi/_helpers.py                77      2     42      0    98%   17-18
google/cloud/bigquery/dbapi/connection.py              15      0      2      0   100%
google/cloud/bigquery/dbapi/cursor.py                 128      2     36      0    99%   21-22
google/cloud/bigquery/dbapi/exceptions.py              11      0      0      0   100%
google/cloud/bigquery/dbapi/types.py                   22      0      0      0   100%
google/cloud/bigquery/encryption_configuration.py      28      0      4      0   100%
google/cloud/bigquery/enums.py                         31      0      2      0   100%
google/cloud/bigquery/exceptions.py                     2      0      0      0   100%
google/cloud/bigquery/external_config.py              314      0     30      0   100%
google/cloud/bigquery/job.py                         1373      0    236      0   100%
google/cloud/bigquery/magics.py                       192      0     69      0   100%
google/cloud/bigquery/model.py                        158      0     24      0   100%
google/cloud/bigquery/query.py                        238      0     72      0   100%
google/cloud/bigquery/retry.py                         13      0      4      0   100%
google/cloud/bigquery/routine.py                      195      0     30      0   100%
google/cloud/bigquery/schema.py                       114      0     42      0   100%
google/cloud/bigquery/table.py                        760      0    244      0   100%
tests/unit/__init__.py                                  0      0      0      0   100%
tests/unit/enums/__init__.py                            0      0      0      0   100%
tests/unit/enums/test_standard_sql_data_types.py       33      0      8      0   100%
tests/unit/helpers.py                                   8      0      0      0   100%
tests/unit/model/__init__.py                            0      0      0      0   100%
tests/unit/model/test_model.py                        125      0      0      0   100%
tests/unit/model/test_model_reference.py               72      0      0      0   100%
tests/unit/routine/__init__.py                          0      0      0      0   100%
tests/unit/routine/test_routine.py                    114      0      0      0   100%
tests/unit/routine/test_routine_argument.py            47      0      0      0   100%
tests/unit/routine/test_routine_reference.py           72      0      0      0   100%
tests/unit/test__helpers.py                           617      0      6      0   100%
tests/unit/test__http.py                               68      0      0      0   100%
tests/unit/test__pandas_helpers.py                    435      0     40      0   100%
tests/unit/test_client.py                            3238      4    104      4    99%   7449->7450, 7450, 7499->7500, 7500, 7541->7542, 7542, 7586->7587, 7587
tests/unit/test_dataset.py                            497      0     20      0   100%
tests/unit/test_dbapi__helpers.py                     116      0     10      0   100%
tests/unit/test_dbapi_connection.py                    73      0      0      0   100%
tests/unit/test_dbapi_cursor.py                       298      0     14      0   100%
tests/unit/test_dbapi_types.py                         18      0      0      0   100%
tests/unit/test_encryption_configuration.py            75      0      0      0   100%
tests/unit/test_external_config.py                    204      0      0      0   100%
tests/unit/test_job.py                               3432      0    132      0   100%
tests/unit/test_magics.py                             748      0      4      0   100%
tests/unit/test_query.py                              625      0      4      0   100%
tests/unit/test_retry.py                               38      0      0      0   100%
tests/unit/test_schema.py                             360      0      8      0   100%
tests/unit/test_signature_compatibility.py             20      0      0      0   100%
tests/unit/test_table.py                             2089      0    100      0   100%
-----------------------------------------------------------------------------------------------
TOTAL                                               18586     10   1865      4    99%

[plamut]

@shollyman The coverage is aggregated across all unit test runs, and that final number is then checked (needs to be 100%).

There are several if-py2.7-else compatibility checks in the code, but since the Python 2.7 unit tests session encountered an error, it did not contribute to the total coverage and the coverage check failed.

It's not related to the changes here, however, as it also happens on master. Some dependencies do not build anymore on older Python versions, will investigate.

Update:
A new version of llvmlite (an indirect dependency) was released three days ago, breaking the installation on Python 2.7 and 3.5. A fix is available: #79.

[plamut]

Generally looks good. I had a few comments, but mostly nits - feel free to dismiss those if you find them too insignificant.

Adding a system test or two makes sense, indeed, awaiting that.

[shollyman]

There's a problem with the integration testing on the datacatalog side, tracking this internally as 154544391

[shollyman]

That said, it seems like you can avoid the integration and simply supply policy tags that match the defined resource pattern, so I'll add a quick test to do so and we can revisit once the datacatalog API issues are resolved.

[plamut]

We missed one deepcopy, sorry for not mentioning it in the initial pass, while the rest looks good. Thanks for the quick update!

Looking forward to that quick test and we should be ready to merge then.

[plamut]

Looks good. The several comments I made during the review have already been fixed in subsequent commits, and what's left is an edge case that seems to not be covered by a test yet.

[shollyman]

After wobbling a bit more, I decided that the right thing here was to make policytags immutable as it lives inside the immutable schemafield entities. Hence, ripped out the setter, and switch to/from api repr to convert between tuple and list forms.

[plamut]

Making the object immutable sounds sensible, and the changes look good. 👍

I just spotted a comment in one of the docstrings that appears to be redundant/misleading, but that's about it.