Unpin CI Python version for AiiDA tests

[ml-evs]

Closes #401

[codecov]

Codecov Report

Merging #472 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #472   +/-   ##
=======================================
  Coverage   91.65%   91.65%           
=======================================
  Files          60       60           
  Lines        2828     2828           
=======================================
  Hits         2592     2592           
  Misses        236      236           

Flag	Coverage Δ
#unittests	91.65% <ø> (ø)

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[CasperWA]

Thanks @ml-evs