Old installed version of Cython is used

[jdemeyer]

On a machine which had on older version of Cython installed but which was upgraded recently after #20218 was merged:

$ ls -ld local/lib/python/site-packages/Cython*
drwxrwxr-x 12 jdemeyer jdemeyer 4096 Sep  6 21:23 local/lib/python/site-packages/Cython
drwxrwxr-x  5 jdemeyer jdemeyer 4096 Sep  6 21:22 local/lib/python/site-packages/Cython-0.23.3-py2.7-linux-i686.egg
drwxrwxr-x  2 jdemeyer jdemeyer 4096 Sep  6 21:14 local/lib/python/site-packages/Cython-0.24.1-py2.7.egg-info

>>> import Cython
>>> Cython.__version__
'0.23.3'
>>> import sys
>>> [x for x in sys.path if 'Cython' in x]
['/home/jdemeyer/sage-git/local/lib/python2.7/site-packages/Cython-0.23.3-py2.7-linux-i686.egg']

This causes breakage when building the Sage library.

Steps to reproduce:

1. ./sage --sh -c 'easy_install cython==0.23.3' # Install an older version of Cython.

2. ./sage -f cython # Rebuild the Cython in Sage. This should uninstall the old version but it doesn't.