Ensure consistent ordering of imports

[cthoyt]

Summary

This PR adds configuration to the lint environment in the tox.ini configuration to run isort, a tool for automatically sorting your imports in a consistent and deterministic way. It also adds some configuration to make it play nice with the black code formatter and an entry into the flake8 configuration so CI checks that isort was applied.

• [x] Code passes all tests
• [x] Unit tests provided for these changes
• [x] Documentation and docstrings added for these changes

Changes

1. Add isort to lint environment
2. Add isort configuration in pyproject.toml
3. Apply isort

[codecov-commenter]

Codecov Report

Merging #25 (28ed97f) into main (49b4171) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
- Coverage   99.87%   99.87%   -0.01%     
==========================================
  Files          15       15              
  Lines         787      770      -17     
==========================================
- Hits          786      769      -17     
  Misses          1        1              

Impacted Files	Coverage Δ
rexmex/dataset.py	100.00% <ø> (ø)
rexmex/__init__.py	100.00% <100.00%> (ø)
rexmex/metrics/__init__.py	100.00% <100.00%> (ø)
rexmex/metrics/ranking.py	100.00% <100.00%> (ø)
rexmex/metrics/rating.py	100.00% <100.00%> (ø)
rexmex/metricset.py	98.33% <100.00%> (-0.13%)	:arrow_down:
rexmex/scorecard.py	100.00% <100.00%> (ø)
rexmex/utils.py	100.00% <100.00%> (ø)
tests/integration/test_aggregation.py	100.00% <100.00%> (ø)
tests/unit/test_dataset.py	100.00% <100.00%> (ø)
... and 3 more

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[cthoyt]

Not really sure what's going on with the code coverage on the diff here