Define pkg as a PackageURL class attribute

[jaimergp]

This will allow library users to subclass PackageURL and redefine the scheme to create PURL-like derivatives. (e.g. for the dep: idea described here):

class DependencyURL(PackageURL):
    SCHEME = "dep"

Which can be used in the same way and happily accepts PEP440-like version ranges in the version field 🥳:

>>> dep = DependencyURL.from_string("dep:pypi/requests@>=2.0")
>>> dep
DependencyURL(type='pypi', namespace=None, name='requests', version='>=2.0', qualifiers={}, subpath=None)