Just use setuptools_scm as prescribed. Fixes #28.

In #28 (closed), I discovered an issue with the technique I proposed in !9 (merged) - namely that the early import of setuptools_scm can cause issues if setuptools_scm isn't present, which it might not be in some cases such as where tox is inspecting the package for its name.

This commit simplifies the code and relies on the plugin functionality in setuptools to perform the same logic. This approach has been long proven and stable, so I recommend it instead. Is there any disadvantage to this approach?