Drop gmpy support in favor of gmpy2

[maximlt]

Closes #661

#971 added gmpy2 in the pixi.toml file instead of gmpy, which are two different projects (the former being maintained while the latter hasn't received an update since 2013). The test suite wasn't updated accordingly so the gmpy-related tests were all skipped (attempting to import gmpy instead of gmpy2).

This MR lets us "officially" drop support of the gmpy Python library (which probably no one used with Param?) in favor of gmpy2.

I'm not entirely sure Param should keep supporting that but ensuring the test suite passes with gmpy2 wasn't too much work. I'd be okay if someone pushes to deprecate it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (aa3315a) to head (982d8d1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1026      +/-   ##
==========================================
+ Coverage   87.25%   87.27%   +0.02%     
==========================================
  Files           9        9              
  Lines        4928     4928              
==========================================
+ Hits         4300     4301       +1     
+ Misses        628      627       -1     

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[jbednar]

Looks good to me. Supporting gmpy2 is important for anyone using param for a simulator, where exact arithmetic is necessary to avoid subtle bugs, so I'm glad this is being updated.