Adopt split repo: _python-pubsub_

[vchudnov-g]

Migrate the split-repo https://github.com/googleapis/python-pubsub to https://github.com/googleapis/google-cloud-python.

The migration readiness criteria are the following, which we track via GitHub labels on this issue. These criteria apply to the split repo we are migrating from:

• No open issues
• No open PRs
• No handwritten samples
• No system tests
• No client-specific customizations

[parthea]

I've marked this issue as blocked while we determine how to migrate the python-pubsub repository to the mono repository.

Note: We should coordinate this effort across various languages. The https://github.com/googleapis/nodejs-pubsub and https://github.com/googleapis/java-pubsub repositories have not migrated to their corresponding mono repositories.

The packages aren't present here:
https://github.com/googleapis/google-cloud-node/tree/main/packages
https://github.com/googleapis/google-cloud-java

[parthea]

Closing as the migration is now complete and we won't be migrating these remaining repositories. We can re-open this issue if we want to onboard this repository to the monorepo.