Overinstall mpl release candidate3v11v0 release candidate

[pp-mo]

Try mpl 3v11 release candidate.
This is a bit awkward because the RC is apparently not included on the anaconda conda-forge channel "conda-forge/label/matplotlob_rc", even though previously versions have been.

So I hacked noxfile.py to over-install it from PyPI.
Checking the feedstock changes, it looks like no dependencies or versions have changed from the 3v10 ones,
which is lucky

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.12%. Comparing base (3f1aa5d) to head (8128aa1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7078   +/-   ##
=======================================
  Coverage   90.12%   90.12%           
=======================================
  Files          91       91           
  Lines       24951    24951           
  Branches     4683     4683           
=======================================
  Hits        22488    22488           
  Misses       1684     1684           
  Partials      779      779           

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[pp-mo]

Closed in favour of #7115