chore: Reduced python version of the client to 3.6

[florianriche]

We migrate to python 3.7 for the API. At the same time we migrated the client python version while it is not required and created a bug.

This PR downgrade the python version of the client package back to python 3.6

[codecov]

Codecov Report

Merging #163 (7159921) into master (86a9513) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #163   +/-   ##
=======================================
  Coverage   91.35%   91.35%           
=======================================
  Files          34       34           
  Lines        1111     1111           
=======================================
  Hits         1015     1015           
  Misses         96       96           

Flag	Coverage Δ
unittests	91.35% <ø> (ø)

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[frgfm]

Thanks!