Add username support for PyPI

[TedTschopp]

PyPI username / owner namespace validation

Category: dev

Goal: Add a validator to check whether a given username/owner exists on PyPI (useful for claiming a consistent package namespace).

Ideas to explore:

• PyPI does not have a super-clear per-user profile URL, but owners can often be inferred from simple endpoints or search filters.
• One approach:
  • Try https://pypi.org/user/{username}/ and inspect whether it returns a distinct status or content for existing vs non-existing users.

Logic (to confirm):

• If 200 vs 404 works, use status_validate().
• Otherwise, use generic_validate() and parse the page text.

Implementation notes:

• Add dev/pypi.py.
• Implement validate_pypi(username: str).
• Keep behavior consistent with other dev platforms (Available/Taken/Error).