Triggered by the recent discussion on deprecating matplotlib.verbose.

This ensures that a deprecation warning is triggered just by trying to
access matplotlib.verbose (or importing it). The implementation idea is
based on njsmith's metamodule, but without the hacks needed to support
Py<3.5.

Py3.7+ will provide instance-__getattr__ on all classes by default,
making the_DeprecatorModuleType patching unnecessary (but the warning
emitter would still need to be implemented, so there is actually fairly
limited improvement from that).

Just putting this here to show that it can be done... attn@jklymak

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is PEP 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