.gitignore: Add mypyc section + pmb: Enable C4 checks

Newbyterequested to merge
newbyte/minor-cleanups into master

When building pmbootstrap with mypyc, this directory is created. While it isn't possible to successfully build pmbootstrap with mypyc yet, I experiment with it every once in a while and it is nice to have this folder ignored for that reason.

Allows for some simplifications.

Code changes were automatically done with Ruff.

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