adjust type hint for filename parameter in open_file to also take an os.PathLike

[danwuSBU]

Underlying functions the parameter filename is passed to has the type str | os.PathLike, but filename accepts only str. Changed the filename type hint to reflect this so that type checking won't complain if, for example, a user passes in a pathlib.Path.

Closes #2717

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.