Switching to importlib resources

[IAlibay]

Fixes #4055

Still some work to be done here (mostly getting rid of __all__ and transferring the information across), but I thought I'd get this in now and let folks comment early since this one is likely to be contentious.

Notes:

• Because the old pkg_resources approach we took generated strings, we can't just use the Path objects directly, hence all the str calls (there's probably a nicer way to do this, I spent all of ~ 10 mins on this).

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[mattwthompson]

Following out of self-interest as I need to make parallel changes in a few places

hence all the str calls

There's also Path.as_posix() which I use because it feels like it might be safer, but I can't find any evidence of that. It's been synonymous with str(Path) in every case I've run into on Unix-like systems ... not sure how much Windows support complicates things as I don't make an effort to do that.

https://docs.python.org/3/library/pathlib.html#pathlib.PurePath.as_posix

[IAlibay]

So this code won't work with py3.8. There are ways to make it backwards compatible with py3.8, but honestly given that NEP29 means we ditch py3.8 on the 14th of April, I would suggest just pushing the 2.5.0 release until then and using the modern API for dealing with resources?

[github-actions]

Linter Bot Results:

Hi @IAlibay! Thanks for making this PR. We linted your code and found the following:

Some issues were found with the formatting of your code.

Code Location	Outcome
main package	✅ Passed
testsuite	⚠️ Possible failure

Please have a look at the darker-main-code and darker-test-code steps here for more details: https://github.com/MDAnalysis/mdanalysis/actions/runs/5065655276/jobs/9094482219

---

Please note: The black linter is purely informational, you can safely ignore these outcomes if there are no flake8 failures!

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (374f0e9) 93.61% compared to head (b02b4f0) 93.61%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4056   +/-   ##
========================================
  Coverage    93.61%   93.61%           
========================================
  Files          192      192           
  Lines        25146    25146           
  Branches      4058     4058           
========================================
  Hits         23540    23540           
  Misses        1089     1089           
  Partials       517      517           

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[IAlibay]

Spurious codecov failure aside - this seems to be working fine now.

@MDAnalysis/coredevs can I get a couple of extra eyes on a final review here please? This is critical for 2.5.0.

[orbeckst]

Thanks to you and sed!