ENH: Fix python source URL.

The Python tarball http URL sends a 301 Moved Permanently response to
the https version.

Placing the real URL makes errors such as:

-- downloading...
     src='http://www.python.org/ftp/python/2.7.3/Python-2.7.3.tgz'
     dst='/home/matt/bin/Slicer-GCC-MinSizeRel/Python-2.7.3.tgz'
 timeout='none'
CMake Error at
python-source-prefix/src/python-source-stamp/download-python-source.cmake:9
(file):
  file DOWNLOAD HASH mismatch

   for file: [/home/matt/bin/Slicer-GCC-MinSizeRel/Python-2.7.3.tgz]
   expected hash: [2cf641732ac23b18d139be077bd906cd]
     actual hash: [d41d8cd98f00b204e9800998ecf8427e]
          status: [1;"unsupported protocol"]

More informative (HTTPS is not supported).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23097 3bd1e089-480b-0410-8dfb-8563597acbee