(and update the outdated pyplot.py).

Closes #3701.
@tacaswell convinced me that a boilerplate.py-generated pyplot is nicer to introspection tools.
It's also a bit tricky to run boilerplate.py as part of setup.py (as suggested in #3701) because it can only be run at a point where _axes.py is actually importable (for introspection), but for example when building a sdist _axes.py is never importable because that never builds the extension modules.

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