Simplify FileList._safe_path for PathLike support

[jaraco]

• In FileList.append
• Refactor _safe_path into three functions. Remove support for bytes. Allow for path to be PathLike.

Summary of changes

Supports pypa/distutils#237.

Pull Request Checklist

• [x] Changes have tests
• [x] News fragment added in newsfragments/. (See documentation for details)

[jaraco]

I wonder if anyone is still dependent on the support for bytes decoding. I guess we should either retain compatibility or declare the change as backward-incompatible. I'll do the latter; we can back out the change (restore support for bytes) if it turns out to cause any trouble.