plugins.handlers: PyFilePlugin version falls back to file's mtime

[dgw]

Description

Alternative to the suggested implementation from #2498, putting this fallback in the plugin machinery instead of a special display-only case in version.

PyFilePlugin keeps track of the modification time when it (re)loads its .py file and uses that to derive a datever if there isn't a "real" version in the module.

Starting as draft… If this idea has legs I'll finish up with any needed test cases.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [ ] No issues are reported by make qa (runs make lint and make test)
  • Didn't run the checks yet. See note above; this is starting as a proof of concept.
• [x] I have tested the functionality of the things this change touches
  • Yes, but just manually.