Make SimplePath path parameters positional-only

[gaborbernat]

Fixes #542.

SimplePath names the joinpath and __truediv__ parameter other without marking it positional-only, so the typing spec requires an implementation to accept other=.... Typeshed names the matching pathlib.Path parameters key and *other, so Path fails the protocol under checkers that compare parameter names:

import pathlib

from importlib_metadata import PathDistribution

PathDistribution(pathlib.Path('x'))

ty 0.0.59 explains it:

info: type `Path` is not assignable to protocol `SimplePath`
info: └── protocol member `__truediv__` is incompatible
info:     └── the parameter named `key` does not match `other` (and can be used as a keyword parameter)

pyrefly 1.1.1 reports the same. mypy 2.1.0 accepts it, which is why the docstring's claim that SimplePath is "A minimal subset of pathlib.Path" has held up until now.

Adding / to both members lets Path satisfy the protocol under ty and pyrefly, and leaves mypy passing. I verified all three against importlib_metadata 9.0.0 with the parameters patched, and checked each protocol member against Path in isolation: joinpath and __truediv__ were the only failures.

ruff format, ruff check and mypy pass. tox -e py fails to collect on this branch, but it fails the same way on a pristine main checkout, so that is unrelated.

pypa/build ran into this while moving off mypy (pypa/build#1135).