Pubsub: pick up fixes to GAPIC generator.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/pubsub/artman_pubsub.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:90f9d15e9bad675aeecd586725bce48f5667ffe7d5fc4d1e96d51ff34304815b
Status: Image is up to date for googleapis/artman:latest
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/pubsub-v1.
synthtool > Replaced 'from google.cloud import pubsub_v1' in tests/unit/gapic/v1/test_publisher_client_v1.py.
synthtool > Replaced ' pubsub_v1' in tests/unit/gapic/v1/test_publisher_client_v1.py.
synthtool > Replaced 'from google.cloud import pubsub_v1' in tests/unit/gapic/v1/test_subscriber_client_v1.py.
synthtool > Replaced ' pubsub_v1' in tests/unit/gapic/v1/test_subscriber_client_v1.py.
synthtool > Replaced '# The name of the interface for this client. This is the key used to' in google/cloud/pubsub_v1/gapic/subscriber_client.py.
synthtool > Replaced '# The name of the interface for this client. This is the key used to' in google/cloud/pubsub_v1/gapic/publisher_client.py.
synthtool > Replaced 'import google.api_core.gapic_v1.method\n' in google/cloud/pubsub_v1/gapic/publisher_client.py.
synthtool > Replaced 'DESCRIPTOR = _MESSAGESTORAGEPOLICY,\n\\s+__module__.*\n\\s+,\n\\s+__doc__ = """' in google/cloud/pubsub_v1/proto/pubsub_pb2.py.
synthtool > No replacements made in google/cloud/pubsub_v1/gapic/subscriber_client.py for pattern subscription \(str\): The subscription whose backlog .*
(.*
)+?\s+Format is .*, maybe replacement is not longer needed?
synthtool > Cleaned up 0 temporary directories.

[tseaver]

Includes fixes from these PRs:

• Remove pending deprecation spew from unit tests. gapic-generator#2407
• Python: expose 'channel' as a property of the transport. gapic-generator#2396