BigQuery: Fix coverage in 'types.py' (via synth).

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/bigquery/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:28d4271586772b275cd3bc95cb46bd227a24d3c9048de45dccdb7f3afb0bfba9
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/bigquery/artman_bigquery_v2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/bigquery/v2/model_reference.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2/google/cloud/bigquery_v2/proto/model_reference.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/bigquery/v2/model.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2/google/cloud/bigquery_v2/proto/model.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/bigquery/v2/standard_sql.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2/google/cloud/bigquery_v2/proto/standard_sql.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2/google/cloud/bigquery_v2/proto.
synthtool > Replaced '"""Attributes:' in google/cloud/bigquery_v2/proto/model_pb2.py.
synthtool > Replaced '[“”]' in google/cloud/bigquery_v2/proto/model_pb2.py.
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/bigquery/.nox/blacken
pip install black
black docs google samples tests noxfile.py setup.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/model_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/model_reference_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/model_reference_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/standard_sql_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/standard_sql_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/bigquery/google/cloud/bigquery_v2/proto/model_pb2.py
All done! ✨ 🍰 ✨
8 files reformatted, 82 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.