Update to run code coverage on GH Action

[jdebacker]

This PR updates the build_and_test GH Action to produce code coverage reports.

It also tries to run the pycodestyle checks.

[codecov-io]

Codecov Report

Merging #689 (7a900d9) into master (67847fc) will decrease coverage by 0.88%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #689      +/-   ##
==========================================
- Coverage   82.52%   81.64%   -0.89%     
==========================================
  Files          48       23      -25     
  Lines        7634     3889    -3745     
==========================================
- Hits         6300     3175    -3125     
+ Misses       1334      714     -620     

Flag	Coverage Δ
unittests	81.64% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
ogusa/aggregates.py	100.00% <ø> (+3.90%)	⬆️
ogusa/firm.py	100.00% <ø> (ø)
ogusa/fiscal.py	100.00% <ø> (ø)
ogusa/output_plots.py	97.47% <ø> (ø)
ogusa/output_tables.py	96.44% <ø> (+7.50%)	⬆️
ogusa/parameter_plots.py	70.56% <ø> (ø)
ogusa/parameters.py	84.74% <ø> (ø)
ogusa/tax.py	99.35% <ø> (ø)
ogusa/TPI.py	94.95% <100.00%> (+19.87%)	⬆️
ogusa/constants.py	100.00% <100.00%> (ø)
... and 30 more

[jdebacker]

Now pinning sphinxcontrib-bibtex version to avoid failure to build Jupyterbook, per solution suggested here.

[rickecon]

@jdebacker . What do you think about the codecov diff hit target failing? It looks like the coverage is somehow looking the tests folder. I don't understand this.

[rickecon]

@jdebacker . Is this OK with 81.64% codecov/project? I am fine with it. If so, then I am ready to merge this.

[jdebacker]

@rickecon Yes, I could spend some more time adding coverage with more tests, but let's merge this now. We need CI testing working for other PRs. Thanks.