Adds a basic stub file generator

[davidselassie]

• Adds stubgen
• Moves PyO3 inner module to _bytewax
• stubgen no longer emits trailing whitespace in docstrings
• Fixes all Python docstrings in Rust code to have proper formatting
• Topo sorts stubgen output so subclasses always work
• Makes stubgen deps depend on Python version; we don't need them in >= 3.9
• Adds bytewax._bytewax stub file; will need to be re-generated on re-build

[whoahbot]

I'm still not seeing the correct signature for any of the rust-based classes after this change. Adding the stub for __init__ with the same signature as the newly-added __new__ stub fixed it for me.

[davidselassie]

Looks like there's something up with pylsp or Jedi for which only overriding __new__ does not cause the signature for the class to show correctly. I changed the stubgen to "rewrite" the signature for __new__ to __init__ which does display correctly in pylsp.

[davidselassie]

I think the better way to think about this is: every class defined in Python's signature is derived from it's __init__. Native classes have a way of defining it separately. But since we're translating from native to Python (the stub), we need to move that information from the class itself to the __init__.