Use Python 3.10 to build docs

[jwodder]

The docs builds have been failing in CI for a while, always(?) due to pip giving up after excessive backtracking while trying to resolve dependencies. I tested out the following failing command from a recent CI run:

pip install 'anys~=0.2' 'bidsschematools~=0.7.0' click-didyoumean 'click>=7.1' coverage 'dandischema<0.12,>=0.9.0' 'etelemetry>=0.2.2' fasteners 'fscacher>=0.3.0' 'hdmf!=3.14.4,!=3.5.0' humanize 'interleave~=0.1' joblib 'keyring!=23.9.0' keyrings.alt 'nwbinspector!=0.4.32,>=0.4.28' opencv-python packaging platformdirs pycryptodomex 'pydantic~=2.0' 'pynwb!=1.1.0,!=2.3.0,>=1.0.3' 'pyout!=0.6.0,>=0.5' pytest pytest-cov pytest-mock pytest-rerunfailures pytest-timeout python-dateutil 'requests~=2.20' 'responses!=0.24.0,!=0.25.5' 'ruamel.yaml<1,>=0.15' semantic-version tenacity 'urllib3>=2.0.0' vcrpy 'yarl~=1.9' 'zarr_checksum~=0.4.0' 'zarr~=2.10'

... in Docker images of Python 3.9, 3.10, 3.11, & 3.13, and 3.9 (the version our docs CI uses) was the only one that went into excessive backtracking. I suspect that something in our dependency tree dropped support for Python 3.9 in a recent version, but I'm not going to go digging to try to find out what.

Strangely, this does not explain why other CI runs using Python 3.9 aren't failing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.43%. Comparing base (4d0d85b) to head (6e9e6e1).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1567      +/-   ##
==========================================
+ Coverage   88.34%   88.43%   +0.08%     
==========================================
  Files          78       78              
  Lines       10735    10735              
==========================================
+ Hits         9484     9493       +9     
+ Misses       1251     1242       -9     

Flag	Coverage Δ
unittests	88.43% <ø> (+0.08%)	⬆️

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[yarikoptic]

wonderful, thank you!