Update URL to PyPI (hopefully)

[stevepiercy]

• Partially resolve Update PyPI URL #1243. There may be other places where the URL for PyPI needs to change from https://pypi.python.org/ to https://pypi.org/

[pradyunsg]

Another place:

pythondotorg/docs/source/administration.rst

Line 9 in 0d99645

Navigation on the site is managed by the `Sitetree <https://pypi.python.org/pypi/django-sitetree>`_ application. The hierarchy should be fairly obvious. The biggest gotcha is when defining the URLs.

(9th line)

[stevepiercy]

This search found a couple more instances, but I don't know whether I should submit commits for them. Please let me know:
https://github.com/python/pythondotorg/search?utf8=%E2%9C%93&q=%22pypi.python.org%22&type=

[berkerpeksag]

Thank you and I'm sorry for missing this PR earlier.