gh-159799: Require pathlib.[Pure]Path for file paths in the PEG generator (#159800)

The pegen entry points now take `pathlib.Path` objects instead of
strings, and the command-line tools (`python -m pegen`, `keywordgen`,
`first_sets`, `grammar_visualizer`) parse their path arguments into
`Path` objects.

The generators' `generate` and `prepare` take a `pathlib.PurePath`
since they only use the file's name (for the "generated by" comment
in the output) and callers pass names that are not files, such as
`"<string>"`.