Revert Matplotlib pinning changes

[nnansters]

Try reverting to the older situation (see 5493766dfdb60315f0ec3a2e3addb6d6836f36d7). It was apparently causing issues in the Conda package build process.

The changes were added to deal with suddenly broken builds on our end.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 76.98%. Comparing base (8bffbcd) to head (9c7a9a9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #403   +/-   ##
=======================================
  Coverage   76.98%   76.98%           
=======================================
  Files         108      108           
  Lines        9267     9267           
  Branches     1656     1656           
=======================================
  Hits         7134     7134           
  Misses       1674     1674           
  Partials      459      459           

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