Deprecate Python 3.7, switch to Python 3.11 in CI

[mryab]

Python 3.7 has reached end-of-line status almost a year ago, which means that we do not need to support it. This PR removes the code that was used to support it (apart from the MPFuture implementation) and switches Actions runners to use Python 3.11 by default

[justheuristic]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.72%. Comparing base (d20e810) to head (cdb87a3).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #621      +/-   ##
==========================================
- Coverage   85.39%   84.72%   -0.68%     
==========================================
  Files          81       81              
  Lines        8006     7987      -19     
==========================================
- Hits         6837     6767      -70     
- Misses       1169     1220      +51     

Files	Coverage Δ
hivemind/utils/asyncio.py	95.34% <100.00%> (-4.66%)	⬇️
hivemind/utils/mpfuture.py	94.14% <100.00%> (+0.81%)	⬆️
hivemind/averaging/matchmaking.py	85.16% <50.00%> (-0.21%)	⬇️

... and 51 files with indirect coverage changes