Asset: pick up fixes in GAPIC generator.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/asset/artman_cloudasset_v1beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
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Status: Downloaded newer image for googleapis/artman:latest
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/asset-v1beta1.
synthtool > Replaced 'from google.iam.v1 import policy_pb2 as' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced 'from google.iam.v1 import iam_policy_pb2_grpc as google_dot_iam_dot_v1_dot_policy__pb2' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced "_ASSET.fields_by_name\\['iam_policy'\\].message_type = google_dot_iam_dot_v1_dot_policy__pb2._POLICY" in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Replaced '          The full name of the asset. For example: ``//compute.googleapi\n          s.com/projects/my_project_123/zones/zone1/instances/instance1`\n          `. See `Resource Names <https://cloud.google.com/apis/design/r\n          esource_names#full_resource_name>`__ for more information.\n' in google/cloud/asset_v1beta1/proto/assets_pb2.py.
synthtool > Cleaned up 0 temporary directories.

[tseaver]

Includes fixes from these PRs:

• Remove pending deprecation spew from unit tests. gapic-generator#2407 (closing PendingDeprecationWarning spew from generated tests gapic-generator#2389)
• Python: expose 'channel' as a property of the transport. gapic-generator#2396 (for PubSub: Errno 24: too many open files with multiple publishers #5523 and dupes).

Unfortunately, the enum-order-instability problem still exists.