Update PyPI URL

[stevepiercy]

There are many instances of https://pypi.python.org/ in the code, but I'm not sure what they all do. The new URL is https://pypi.org/. Making updates would save a redirect.

Anyway, the one I wanted to update is that which appears in the uppermost navigation bar. I think it's this one based on its context, but I'm not positive.
https://github.com/python/pythondotorg/blob/master/cms/views.py#L6

If someone can verify, I can update that one. I don't know whether others need to be updated.

[MarkMangoba]

@stevepiercy thanks for the report! Can you open up a PR for this?

[stevepiercy]

Done! See #1245.