NEP29, raise Python to 3.9

[IAlibay]

Towards 2.5.0 release

We won't release until the 14th (at least I'm not planning to do a release over Easter holidays), so I think we can safely say we'll bump things up to Python 3.9 in line with NEP29.

PR Checklist

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

---

📚 Documentation preview 📚: https://readthedocs-preview--4115.org.readthedocs.build/en/4115/

[github-actions]

Linter Bot Results:

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

There are currently no issues detected! 🎉

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (fd978d2) 93.59% compared to head (228f892) 93.59%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4115   +/-   ##
========================================
  Coverage    93.59%   93.59%           
========================================
  Files          192      192           
  Lines        25134    25134           
  Branches      4056     4056           
========================================
  Hits         23524    23524           
  Misses        1092     1092           
  Partials       518      518           

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

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

[orbeckst]

lgtm, thanks @RMeli for doing the actual reviewing

[orbeckst]

Presumably, a bunch of secondary docs might need updating as well but I suppose we do this when it shows up.

[orbeckst]

Azure 3.9 32bit is unhappy

ERROR: Could not find a version that satisfies the requirement h5py (from versions: none)

[IAlibay]

yeah turns out that h5py hasn't released 32bit binaries for py3.9+

I'm just going to skip that pip install in that case.

[IAlibay]

Presumably, a bunch of secondary docs might need updating as well but I suppose we do this when it shows up.

afaik nothing in this repo, mostly userguide... which needs a good update

[tylerjereddy]

LGTM as well, pyproject.toml is always the trickiest to be sure about/parse, but seems sensible I think.

[tylerjereddy]

NumPy and SciPy have also dropped 3.8 on their main branches.