ci(gh-tests): change pip version for test jobs

[oboulant]

Some tests do not complete and time out.
We started noting this behaviour in #198
Tests are fine for python v3.8 and v3.9. Nevertheless, for v3.6 and v3.7, jobs stay stuck on the python -m pip install .[test] instruction.

Previously, a specific pip version was enforced because at that time there was also some GH Actions jobs that stayed stuck. See #153

[codecov]

Codecov Report

Merging #204 (ea9d69c) into master (b84df2c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #204   +/-   ##
=======================================
  Coverage   96.59%   96.59%           
=======================================
  Files          40       40           
  Lines         969      969           
=======================================
  Hits          936      936           
  Misses         33       33           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b84df2c...ea9d69c. Read the comment docs.