Adjust PyPI URL to new home of PyPI

Without this, pypi-install is broken.

This patch is important, as without it, the find_tar_gz() function
crashes with:

  File "/usr/lib/python2.7/xmlrpclib.py", line 1312, in single_request
    response.msg,
xmlrpclib.ProtocolError: <ProtocolError for python.org/pypi: 301 Moved Permanently>

This addresses the problem by using the canonical URL of PyPI.

I hereby license this under the same terms of distribution as stdeb itself.