Rename make_srreal_ext to create_extensions.

Use more generic name.  create_extensions can return list in case
of several extension modules.  setup.py is importable without searching
for libraries etc.
Remove dependency_links, use only canonic PyPI data.
Tags must match  v[[:digit:]]  to define version number.

This adopts to conventional version tag naming vN.M[...].
Mark vigorously this needs a fix.
Require the diffpy.Structure dependency.
Update setup.py as per PackageChecklist.

https://github.com/diffpy/diffpy-release/wiki/PackageChecklist#wiki-setuppy
No need to explicitly include setup.cfg.

Does not contain any useful data.  setuptools sdist will include
it no matter what anyway.
Change the default build mode to "fast".

Developers can set build=debug.