Don't clear the category after the initial package.

Don't clear the category after the initial package.

My previous implementation actually set the category to the empty
string, which is definately not what we want.  The current
implementation checks both the specified category and dev-python when
looking for required packages, and defaults to generating all required
packages in the specified category.

The literal set syntax `{a,b,...}` requires Python >= 2.7, but that's
the current default in Portage, so anyone running g-pypi should have
it already.