chore: upgrade gapic-generator-python to 0.39.1 (#64)
chore: upgrade gapic-generator-python to 0.39.1 (#64)

feat(v2beta3, v2beta2): introducing fields: ListQueuesRequest.read_mask, GetQueueRequest.read_mask, Queue.task_ttl, Queue.tombstone_ttl, Queue.stats and introducing messages: QueueStats
feat: add from_service_account_info method to clients
docs(v2beta3): updates to max burst size description
docs(v2beta2): updates to AppEngineHttpRequest description
test: add 3.9 unit tests
docs: fix type references in docstrings
GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples