Add license classifier to setup.py

[dotcs]

This PR adds a license classifier so that the correct license (MIT) appears in the Python Package Index (PyPI) package overview. See PyPI Classifiers List for all possible classifiers.

[codecov-commenter]

Codecov Report

Merging #253 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #253   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files           7        7           
  Lines         387      387           
  Branches       39       39           
=======================================
  Hits          381      381           
  Misses          3        3           
  Partials        3        3           

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