bisect very hard with rcParam changes

[jklymak]

At some point recently we changed rcParam handling so that it makes git bisect require that matplotlib be rebuilt every time a bisect step crosses a change in the rcParam list. This seems a needless pain, and we should just warn.

  import matplotlib as mpl
Traceback (most recent call last):
  File "/Users/jklymak/matplotlib/testlisted.py", line 1, in <module>
    import matplotlib as mpl
  File "/Users/jklymak/matplotlib/lib/matplotlib/__init__.py", line 833, in <module>
    defaultParams = rcsetup.defaultParams = {
  File "/Users/jklymak/matplotlib/lib/matplotlib/__init__.py", line 836, in <dictcomp>
    rcParamsDefault[key]),
  File "/Users/jklymak/matplotlib/lib/matplotlib/__init__.py", line 621, in __getitem__
    return dict.__getitem__(self, key)
KeyError: 'axes.zmargin'

[tacaswell]

or at least have MPLBISECT_MODE=1 env?

[anntzer]

fixed by #19603, I believe.