Ensure edit_constant resets class and instance level parameters

[maximlt]

Fixes #1014
Fixes #931

Regression introduced in Param 2.0.0 in #826 that introduced installing an instance parameter (copied from the class) on __set__ (6ce1ef3). The context manager edit_constant was first getting a handle on all the existing parameters (mix of class and instance level parameters), yielding, and then resetting constant on the parameters already handled, missing that the parameterized object could have been equipped with instance-level parameters that were created with constant=False.

This is fixed in this MR, which also only touches the constant parameters.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.27%. Comparing base (8095bb3) to head (87b16ca).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1015      +/-   ##
==========================================
+ Coverage   87.25%   87.27%   +0.01%     
==========================================
  Files           9        9              
  Lines        4928     4934       +6     
==========================================
+ Hits         4300     4306       +6     
  Misses        628      628              

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[jbednar]

Seems correct to me. Thanks!