unit-3.6 session is failing on Mac

[tswast]

nox > Running session unit-3.6
nox > Re-using existing virtual environment at .nox/unit-3-6.
nox > pip install mock pytest google-cloud-testutils pytest-cov freezegun -c /Users/swast/src/python-bigquery/testing/constraints-3.6.txt
nox > pip install -e .[all] -c /Users/swast/src/python-bigquery/testing/constraints-3.6.txt
nox > Command pip install -e .[all] -c /Users/swast/src/python-bigquery/testing/constraints-3.6.txt failed with exit code 1:
Obtaining file:///Users/swast/src/python-bigquery
Collecting google-api-core[grpc]<2.0.0dev,>=1.23.0
  Using cached google_api_core-1.23.0-py2.py3-none-any.whl (91 kB)
Collecting proto-plus>=1.10.0
  Using cached proto_plus-1.10.0-py3-none-any.whl
Collecting google-cloud-core<2.0dev,>=1.4.1
  Using cached google_cloud_core-1.4.1-py2.py3-none-any.whl (26 kB)
Collecting google-resumable-media<2.0dev,>=0.6.0
  Using cached google_resumable_media-0.6.0-py2.py3-none-any.whl (41 kB)
Requirement already satisfied: packaging>=14.3 in ./.nox/unit-3-6/lib/python3.6/site-packages (from google-cloud-bigquery==2.12.0) (20.9)
Collecting protobuf>=3.12.0
  Using cached protobuf-3.12.0-cp36-cp36m-macosx_10_9_x86_64.whl (1.3 MB)
Collecting google-cloud-bigquery-storage<3.0.0dev,>=2.0.0
  Using cached google_cloud_bigquery_storage-2.0.0-py2.py3-none-any.whl (60 kB)
Collecting grpcio<2.0dev,>=1.32.0
  Using cached grpcio-1.32.0-cp36-cp36m-macosx_10_9_x86_64.whl (3.3 MB)
Collecting pyarrow<4.0dev,>=1.0.0
  Using cached pyarrow-1.0.0-cp36-cp36m-macosx_10_9_intel.whl (11.1 MB)
Collecting pandas>=0.23.0
  Using cached pandas-0.23.0-cp36-cp36m-macosx_10_6_intel.macosx_10_9_intel.macosx_10_9_x86_64.macosx_10_10_intel.macosx_10_10_x86_64.whl (16.8 MB)
ERROR: Could not find a version that satisfies the requirement pyarrow<4.0dev,>=3.0.0 (from google-cloud-bigquery[all])
ERROR: No matching distribution found for pyarrow<4.0dev,>=3.0.0
nox > Session unit-3.6 failed.

I think the bignumeric extra needs to be excluded from all or we need to add some logic to avoid installing that extra from Python 3.6.

Not sure why the Kokoro jobs aren't failing, though.

[tswast]

There are pyarrow 3.0.0 wheels for Python 3.6 https://pypi.org/project/pyarrow/#files

I think this is just a matter of the constraints file trying to force 1.0.0, which we do still want to test with.