Replace sphinx_astropy sphinx extension with pytest-doctestplus

[eslavich]

Description

This PR attempts to resolve the doc build failure due to changes in sphinx_astropy 1.5.0.

Checklist

• Tests

• Documentation

• Change log

• Milestone

• Label(s)

[codecov]

Codecov Report

Merging #6230 (6f1ab0d) into master (d123c28) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6230      +/-   ##
==========================================
+ Coverage   77.69%   77.77%   +0.08%     
==========================================
  Files         402      402              
  Lines       34412    34422      +10     
==========================================
+ Hits        26736    26773      +37     
+ Misses       7676     7649      -27     

Flag	Coverage Δ		*Carryforward flag
nightly	77.69% <ø> (ø)		Carriedforward from d123c28
unit	55.54% <ø> (?)

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/associations/asn_edit.py	66.66% <0.00%> (-0.75%)	⬇️
jwst/associations/lib/process_list.py	73.49% <0.00%> (-0.59%)	⬇️
jwst/fits_generator/template.py	16.98% <0.00%> (-0.27%)	⬇️
jwst/timeconversion/time_conversion.py	10.96% <0.00%> (-0.08%)	⬇️
jwst/datamodels/make_header.py	91.38% <0.00%> (+0.02%)	⬆️
jwst/associations/lib/product_utils.py	94.82% <0.00%> (+0.09%)	⬆️
jwst/associations/lib/constraint.py	93.65% <0.00%> (+0.37%)	⬆️
jwst/regtest/conftest.py	90.90% <0.00%> (+1.21%)	⬆️
jwst/wiimatch/match.py	67.30% <0.00%> (+1.92%)	⬆️
jwst/wiimatch/lsq_optimizer.py	88.42% <0.00%> (+5.26%)	⬆️
... and 3 more

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[jdavies-st]

Looks good.