pytest-doctest-docutils: Fix python files

[tony]

To scan .py files, in lieu of pytest's own docutils being disabled in pytest_configure:

Adds --doctest-docutils-modules (scan doctests in .py files) and --no-doctest-docutils-modules (ignore doctest in .py files)

This should be temporary, ideally we'll find a way to work with pytest doctest without needing to block the plugin

[codecov-commenter]

Codecov Report

Merging #7 (338bb6f) into master (caf7cfe) will increase coverage by 0.92%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   59.02%   59.94%   +0.92%     
==========================================
  Files           9       11       +2     
  Lines         615      724     +109     
==========================================
+ Hits          363      434      +71     
- Misses        252      290      +38     

Impacted Files	Coverage Δ
src/pytest_doctest_docutils.py	40.77% <80.00%> (+3.54%)	⬆️
tests/test_pytest_doctest_docutils.py	100.00% <100.00%> (ø)
src/gp_libs.py	100.00% <0.00%> (ø)
docs/conf.py	59.09% <0.00%> (ø)

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