Fix broken CI

[rly]

Motivation

windows-py310
and X-gallery-python3.8-prerelease are failing in nightly CI builds

Try to resolve the errors by updating requirements.txt and no longer testing Python 3.7 with the pinned dependencies in requirements.txt which severely constrains dependency resolution.

Also update versioneer, make readthedocs use Python 3.9, and other minor infrastructure improvements

How to test the behavior?

Run all CI

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

Merging #1566 (9771300) into dev (6e1f131) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #1566      +/-   ##
==========================================
- Coverage   90.65%   90.65%   -0.01%     
==========================================
  Files          25       25              
  Lines        2494     2493       -1     
  Branches      466      466              
==========================================
- Hits         2261     2260       -1     
  Misses        148      148              
  Partials       85       85              

Flag	Coverage Δ
integration	69.59% <100.00%> (-0.02%)	⬇️
unit	84.15% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pynwb/__init__.py	77.20% <100.00%> (-0.17%)	⬇️

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