fix(bigquery): import Mapping from collections.abc not from collections

[alextford11]

Getting following deprecation warning in Bigquery:

File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/job.py", line 3216, in __iter__
    return iter(self.result())
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/job.py", line 3105, in result
    dest_table = Table(dest_table_ref, schema=schema)
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/table.py", line 335, in __init__
    self.schema = schema
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/table.py", line 400, in schema
    value = _to_schema_fields(value)
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/schema.py", line 284, in _to_schema_fields
    if not isinstance(field, (SchemaField, collections.Mapping)):
  File "/usr/lib/python3.7/collections/__init__.py", line 52, in __getattr__
    DeprecationWarning, stacklevel=2)
DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

[tswast]

When was the abc module added? If not until 3.x, we still need to account for compatibility with Python 2.7 for a little while longer.

[tswast]

Thanks!

Just kicked off the Kokoro CI tests. We can merge, assuming those pass.

[samuelcolvin]

@tswast I know you like importing modules, not classes, but this is the canonical approach, anything else will be very unclear.

[robert-l1]

I believe this update is causing issues with several versions of the BigQuery client class. My images are now throwing:

Traceback (most recent call last):
  File "test.py", line 20, in <module>
    from google.cloud import bigquery
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/__init__.py", line 35, in <module>
    from google.cloud.bigquery.client import Client
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/client.py", line 58, in <module>
    from google.cloud.bigquery import _pandas_helpers
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/_pandas_helpers.py", line 40, in <module>
    from google.cloud.bigquery import schema
  File "/usr/local/lib/python3.7/dist-packages/google/cloud/bigquery/schema.py", line 17, in <module>
    from six.moves import collections_abc
ImportError: cannot import name 'collections_abc' from 'six.moves' (unknown location)

I see the same in 2.7:

    from google.cloud import bigquery
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/bigquery/__init__.py", line 35, in <module>
    from google.cloud.bigquery.client import Client
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/bigquery/client.py", line 58, in <module>
    from google.cloud.bigquery import _pandas_helpers
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/bigquery/_pandas_helpers.py", line 40, in <module>
    from google.cloud.bigquery import schema
  File "/usr/local/lib/python2.7/dist-packages/google/cloud/bigquery/schema.py", line 17, in <module>
    from six.moves import collections_abc
ImportError: cannot import name collections_abc

[tkrassowski]

@robert-l1
Upgrade your six package to newer version. Collections_abc support came in 1.13.1 (benjaminp/six#241).

[neteler]

@robert-l1
Upgrade your six package to newer version. Collections_abc support came in 1.13.1

FWIW: So far six 1.13.0 is the latest release (https://pypi.org/project/six/) and seems to be sufficient.