More support for Paths, and no longer modifies the doctest module.

Improvements

• Reader(shp=, dbf=, shx=) now support pathlib.Paths, and any pathlike object (@mwtoews).

Bug fixes

• PyShp 3 no longer modifies the global doctest module (@JamesParrott).

Code quality

• isort replaced by Ruff check's I rule (@mwtoews).
• mypy --strict used in CI (@JamesParrott).
• LICENSE.TXT re-encoded in UTF-8 (@musicinmybrain).