It was only accessible by modifying the source of transforms.py to set
DEBUG to True (before the module is imported), and anyways broken
because transforms are not hashable in Py3 (so the call to hash()
fails).

Instead move it to a private module.

Example use:

from matplotlib._internal_utils import graphviz_dump_transform
graphviz_dump_transform(plt.gca().transAxes, "/tmp/test.png")

Also, stop tracking transform node children even in debug mode, as it
was only used when dumping the transform but that's not even necessary
-- one can just inspect vars(node) to look for children.

Also fix AffineBase.__eq__ to check that the "other" object has a
get_matrix method, to not raise an AttributeError when comparing with
Bboxes (which don't).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way