setupext currently has a complex machinery to define extension modules.
Essentially, the problem is that numpy may not be installed at the time
the extensions are declared, so we can't call np.get_include() to get
the include paths. So we use a DelayedExtension class and a hook
mechanism to inject the numpy include path later, once it becomes
available.

Instead, we can just declare a dummy extension (so that setuptools
doesn't elide the call to build_ext), then swap in the correct
extensions in build_ext.finalize_options(): at that point, numpy will
have been installed and we don't need any crazy machinery.

Alsocloses#6928 by not attempting to reload numpy anymore.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 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