py3-argon2-cffi-bindings - fix import error, add test.

[smoser]

This module has both a runtime and buildtime dependency on py3-cffi.

The import test is re-added here. It would fail without the fix and passes with it.

Also drop the duplicate 'provides' (py3-) py3-argon2-cffi-bindings == ${{package.name}} == py3-${{vars.pypi-package}}