Add linters and auto formatter

[dmohns]

Apply'ing black and ruff to the repository.

Doing this in a first step, such that we can add the merge commit to .git-blame-ignore-revs file in a second step, see https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view

After that, adding CI to check linting of Pull Requests.

Note: This should be merged using Squash&Rebase

[benmod]

All good except those imports in the init.py file that the linter wrongly thought were unused but are actually used by anybody using the library.

[benmod]

Why removing those imports? They are left at top level so that you can access them importing just the module, if we remove them it will break existing tools using it.

[dmohns]

It raised "unused variable" warnings. I think the recommended approach is to use __all__. Let me try to adapt. Do you have a piece of code, where I can test?

[benmod]

Nothing public but basically you should be able to import all of those variables when importing openpaygo. Like from openpaygo import TokenType should work.

[dmohns]

I personally find it a bit odd, if from openpaygo import TokenType works, but from openpaygo import * and then openpaygo.TokenType doesn't. Using __all__ should be non-breaking. WDYT?

[benmod]

All good now!