Install "test" extras when building docs

[jwodder]

The docs try to document a couple test modules that import pytest, so that needs to be installed when building the docs.

Closes #717.

[codecov]

Codecov Report

Merging #718 (8d832f2) into master (7546b2d) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #718      +/-   ##
==========================================
+ Coverage   83.96%   84.06%   +0.10%     
==========================================
  Files          59       59              
  Lines        5661     5661              
==========================================
+ Hits         4753     4759       +6     
+ Misses        908      902       -6     

Flag	Coverage Δ
unittests	84.06% <ø> (+0.10%)	⬆️

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Impacted Files	Coverage Δ
dandi/dandiapi.py	90.93% <0.00%> (+0.26%)	⬆️
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

woohoo -- RTD is happy, thank you @jwodder!