chore(bigquery): use 3.6 for blacken session

[yoshi-automation]

This PR was generated using Autosynth. 🌈

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:feed210b5723c6f524b52ef6d7740a030f2d1a8f7c29a71c5e5b4481ceaad7f5
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/encryption_config.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/bigquery-v2/google/cloud/bigquery_v2/proto/encryption_config.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 '"""Attributes:' in google/cloud/bigquery_v2/proto/encryption_config_pb2.py.
synthtool > No replacements made in google/cloud/bigquery_v2/proto/*.py for pattern [“”], maybe replacement is not longer needed?
Running session blacken
Session blacken skipped: Python interpreter 3.7 not found.
synthtool > Wrote metadata to synth.metadata.