Python ufmt fixes

[sdwilsh]

Here are the commits I've got to update the devcontainer to use ufmt. This includes a change that will go away if you rebase your commits.

I didn't update the readme because I'm not sure what badge to use here, if I'm being honest.

I also don't think uvx works with vscode's settings, alas, so I have to setup a venv, which ended up being a giant pain to get working.

[ofek]

Are you fine with me removing the manual Python management in favor of using uv which is already installed in the image? It would remove a lot of future maintenance on your part.

[sdwilsh]

As long as it works well with vscode still, I don't really care.

The only potentially complicated part is that the requirements-fmt.txt file needs to exist (it's shared across Python-based projects at Meta), and that's how those deps will need to be managed.

[ofek]

Thanks!