Pypi type specific transformations should match PEP 503

[andrewstein]

The pypi type specific transformations do not match PEP 505. The current code is based of the existing purl spec, but does not take package-url/purl-spec#165 into account.

I have code on my machine, which both demonstrates the issue with real-world tests cases and provides a fix. Unfortunately, the permissions on this repo do not allow me to create a pull request. @jdillon is you could change these permissions, I will push a PR.