[MRG] add required python version to setup.py

[sappelhoff]

closes #314

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Merge checklist

Maintainer, please confirm the following before merging:

• All comments resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• PR description includes phrase "closes <#issue-number>"
• Commit history does not contain any merge commits

[codecov-io]

Codecov Report

Merging #320 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #320   +/-   ##
=======================================
  Coverage   94.79%   94.79%           
=======================================
  Files          11       11           
  Lines        1152     1152           
=======================================
  Hits         1092     1092           
  Misses         60       60

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[sappelhoff]

same for this one @jasmainak please check and merge if all is well.

[jasmainak]

Thanks @sappelhoff !