Python client moved from super repo

[philrz]

What's Changing

This is a move of the Python client to this new repo from its prior location at python/superdb/ in the super repo.

Why

Two reasons I'm aware of:

1. There's been some debate about whether use of pytest is appropriate in the Go-centric super repo, but that seems less of a concern in a separate Python-centric repo.
2. Because installing the client via pip pulls in pyarrow which is a pretty big module, the client's ztests were reported as taking ~22 seconds or more when running local tests in the super repo.

Details

This prepares an approach similar to what we've used for repos like Zui/Brimcap in the past: Tests will run automatically in Actions whenever a PR merges in super, and failure notifications will arrive via typical notifications like email/Slack. If a Dev is doing things they fear may break the client and they want to be proactive, the README describes how they can run the client tests locally, or they can enter their branch ref in the repository_dispatch input in the Actions UI.

Next Steps

If this merges, there's four things I'd plan to do next:

• Add a ref to superdb-python in the repo: matrix in super's notify-downstream-merge.yaml Actions Workflow so the tests will be triggered when super PRs merge
• Update the docs to have it mention this repo
• Delete the python/ directory from the super repo (which will result in the ztests going away too)
• Announce the change in Release Notes and on community Slack