Pin pytest version in requirements-dev, use file_descriptor in tests

[justheuristic]

This fixes the "previous item was not torn down properly" error in CI.

pytest-dev/pytest#9621

[justheuristic]

(copied from discord)

• tests failing was not just random, there was a pattern to it

• pytest failed specifically with codecov enabled - you can see it consistently failed in 5 last failed builds
  

• and it failed specifically on test_mode -> remote_call_many

Here's a proof: removing codecov fixes the issue https://github.com/learning-at-home/hivemind/runs/5124226685?check_suite_focus=true
I'm now investigating if this is fixed by pinning codecov version to be compatible to that pytest

[codecov]

Codecov Report

Merging #454 (8999dd9) into master (2abda6f) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
+ Coverage   84.01%   84.05%   +0.03%     
==========================================
  Files          78       78              
  Lines        7932     7932              
==========================================
+ Hits         6664     6667       +3     
+ Misses       1268     1265       -3     

Impacted Files	Coverage Δ
hivemind/averaging/matchmaking.py	87.50% <0.00%> (ø)
hivemind/averaging/averager.py	88.37% <0.00%> (+0.72%)	⬆️

[mryab]

To conclude, the CI failures for the pytest/codecov jobs are related to pytest-dev/pytest#9621 and pytest-dev/pytest-cov#520; adding these issues here to have backlinks to this PR in their discussion threads