Replace pytest-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 #259 (83b424e) into main (a70e49a) will decrease coverage by 11.78%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #259       +/-   ##
===========================================
- Coverage   58.16%   46.38%   -11.79%     
===========================================
  Files          36       35        -1     
  Lines         753      746        -7     
===========================================
- Hits          438      346       -92     
- Misses        315      400       +85     

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Impacted Files	Coverage Δ
romancal/flatfield/flat_field.py	26.47% <0.00%> (-64.71%)	⬇️
romancal/flatfield/flat_field_step.py	28.00% <0.00%> (-56.00%)	⬇️
romancal/regtest/regtestdata.py	27.04% <0.00%> (-27.56%)	⬇️
romancal/stpipe/core.py	80.00% <0.00%> (-13.34%)	⬇️
setup.py

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[dmggh]

Looks good to me.