mplutils.cpp only provides add_dict_int, which (for us) can be replaced
by PyModule_AddIntConstant (already used in the same fashion in
image_wrapper.cpp).

MAX and CLAMP can be replaced via std::max/std::min, as they are only
used in C++ modules.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).