The Numpy class in setupext.py serves two purposes: declare a
setup_requires and an install_requires on numpy, and provide a
add_flags() (effectively static) method to link extension modules to
numpy.

Instead, we can just merge the setup and install_requires together
with all other install_requires, and move all of them to setup.py
(deleting the machinery to grab setup_requires and install_requires
from multiple places), and make the add_flags() method a free function
(add_numpy_flags()).

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