chore(dependencies): pin matplotlib<=3.9.4 for now

[wpbonelli]

Some of the example scripts have begun to break with matplotlib 3.10. Most of the failures are due to the removal of the collections attribute from ContourSet and related API changes. We will need to rewrite some things to work around this. For reference see the corresponding change from a while ago in flopy.

Two failures however seem to be internal to matplotlib, with pillow being used incorrectly:

Traceback (most recent call last):
    File "/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/matplotlib/animation.py", line 224, in saving
      yield self
    File "/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/matplotlib/animation.py", line 1126, in save
      writer.grab_frame(**savefig_kwargs)
    File "/opt/hostedtoolcache/Python/3.10.15/x64/lib/python3.10/site-packages/matplotlib/animation.py", line 499, in grab_frame
      self._frame.append(im)
  AttributeError: 'PillowWriter' object has no attribute '_frame'. Did you mean: '_frames'?

While we address the former and wait for a resolution for the latter, we can just pin mpl.