chore: Upgraded Python to 3.7

[florianriche]

Since the introduction of #156 , the API can't be deploy because one of the introduced feature is not compatible with python 3.6
See Heroku log stack trace:

This PR upgrade the minimum python version to 3.7

[codecov]

Codecov Report

Merging #159 (38fccfa) into master (e2896f2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   91.43%   91.43%           
=======================================
  Files          34       34           
  Lines        1109     1109           
=======================================
  Hits         1014     1014           
  Misses         95       95           

Flag	Coverage Δ
unittests	91.43% <ø> (ø)

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[frgfm]

Thanks for the fix!