Make codegen deterministic

[DXsmiley]

I checked out the repo and, without changes, ran atproto gen all and unexpectedly ended up with a bunch of modified files. It looks like it's mostly just large chunks being moved around inside atproto/xrpc_client/namespaces/sync_ns.py though.

I suspect the culprit is os.walk but I'm not sure. Unfortunately I think it's a system-dependent thing so I don't quite have the ability to test this thoroughly on my machine.

This doesn't impact library users, but I would like to make a few other changes to the codegen without producing a completely unreadable diff.

[MarshalX]

What should we do to be sure?

[DXsmiley]

If you checkout my branch, run atproto gen all and something changes we'll know there's unfixed problems. If there's no changes then it's hard to say for certain that everything's fixed but it'll give me a lot of confidence.

I think also adding a github action that runs the codegen and checks there's been no changes could also prevent non-determinism being reintroduced in the future. (and should also catch any instances of people making changes and then just forgetting to run the codegen script).

[MarshalX]

Could you implement this GitHub actions workflow in this PR please?

[DXsmiley]

@MarshalX done. I think it needs your manual approval before it'll actually run though.

[MarshalX]

Awesome! Could we run it in matrix on Linux macOS and Windows? To be sure that there is no difference between platforms

[DXsmiley]

done! needs another approval though

[DXsmiley]

made changes in accordance with https://github.com/snok/install-poetry#running-on-windows to try and get poetry working

[MarshalX]

problem of python 3.7. as ez fix (and end of life of python 3.7) let's use python 3.8 here? https://www.mail-archive.com/python-bugs-list@python.org/msg426517.html

[MarshalX]

thank you!