add requirements.txt

[jklynch]

adding requirements.txt to MANIFEST.in

[codecov-io]

Codecov Report

Merging #23 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #23   +/-   ##
=======================================
  Coverage   66.22%   66.22%           
=======================================
  Files           4        4           
  Lines         450      450           
=======================================
  Hits          298      298           
  Misses        152      152

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