Linting and Formatting

[jalving]

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This PR makes many changes because it updates all of the code formatting. The main changes are:

• All src and test files updated using isort and black linting operations.
• setup.cfg requires python >= 3.7
• A doc string for OMLT in the outer __init__.py file
• changes to tox.ini such that github actions SHOULD execute linting checks with flake8 and black.

Note that most linting is ignored. A separate issue will be opened to address flake8 stylistic requirements.

[codecov[bot]]

Codecov Report

Merging #84 (76c790c) into main (bc46b99) will not change coverage. The diff coverage is 90.67%.

@@           Coverage Diff           @@
##             main      #84   +/-   ##
=======================================
  Coverage   94.19%   94.19%           
=======================================
  Files          25       25           
  Lines        1240     1240           
  Branches      192      192           
=======================================
  Hits         1168     1168           
  Misses         42       42           
  Partials       30       30           

Impacted Files	Coverage Δ
src/omlt/__init__.py	100.00% <ø> (ø)
src/omlt/block.py	100.00% <ø> (ø)
src/omlt/io/__init__.py	100.00% <ø> (ø)
src/omlt/neuralnet/__init__.py	100.00% <ø> (ø)
src/omlt/neuralnet/layer.py	90.57% <60.00%> (ø)
src/omlt/io/keras/keras_reader.py	89.47% <83.33%> (ø)
src/omlt/neuralnet/nn_formulation.py	92.92% <87.50%> (ø)
src/omlt/io/onnx_parser.py	95.00% <88.23%> (ø)
src/omlt/formulation.py	92.53% <100.00%> (ø)
src/omlt/gbt/gbt_formulation.py	93.65% <100.00%> (ø)
... and 16 more

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[jalving]

The linting check now gets hit on the Python 3.8 CI tests.

The codecov/patch coverage drops because the black reformatting creates more lines of code for calls we don't test yet. I think it is okay to ignore, but we should look at the warnings and write those tests at some point.

Also note that almost all flake8 checks are ignored. We should make an issue to clean up our code for flake8 style requirements.