maintenance/admin for setup/README/etc.

[jGaboardi]

This PR:

• updates supported Python versions in setup.py
• removes nose / testing suite information from setup.py (see here)
• ensures uniform formatting in README.md
• adds a .txt extension to LICENSE
• updates MANIFEST.in to include itself and the license file

[codecov-io]

Codecov Report

Merging #117 (0d6c873) into master (5f32e05) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #117   +/-   ##
=======================================
  Coverage   81.50%   81.50%           
=======================================
  Files          13       13           
  Lines         600      600           
=======================================
  Hits          489      489           
  Misses        111      111           

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