Handle definitions @import from relative paths on Windows

[khaeru]

Initially as suggested at #1498 (comment), since this continues to affect downstream packages and their CI on Windows.

The actual fix ended up being using pathlib.Path for ImportDefinition.path; this way its str() representation can be compared with those of other Path objects, and they will be consistent on any given OS.

I realized that the pint test suite does not run on Windows; this is one reason the particular manifestation of the bug mentioned at #1498 (comment) was not noticed in #1499. So, I added (probably in the wrong place/form, sorry) a slimmed-down copy of the test-linux jobs, named test-windows, to confirm.

Happy to make any desired adjustments.

• Closes New base units not usable in @context in @import-ed file #1498
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate N/A as in Provide a method to iter the definitions in the order they appear #1499
• Added an entry to the CHANGES file

[khaeru]

The GHA workflows appear to run under my fork (neat), so here's a confirmation of the bug occurring on the Windows runners: https://github.com/khaeru/pint/runs/5969710141?check_suite_focus=true#step:9:161

I'll now remove the "TEMPORARY" commits from the branch.

[khaeru]

Now seeing this:

 E                   ValueError: The order of the files do not match. (expected: C:\Users\runneradmin\AppData\Local\Temp\pytest-of-runneradmin\pytest-0\test_issue14980\def2.txt, found C:\Users\runneradmin\AppData\Local\Temp\pytest-of-runneradmin\pytest-0\test_issue14980\def2.txt)

These are the same files; nothing to do with subdirectories.

Will experiment further!

[khaeru]

The new Windows jobs now pass: https://github.com/khaeru/pint/runs/5969959860?check_suite_focus=true