Fix setuptools issue with cython extensions on python26

When importing setuptools it monkey-patches distutils and replaces the
Extension class. The replaced version converts '.pyx' extensions to '.c'
automatically if it detects that neither Pyrex nor Cython are installed.
However, earlier versions of setuptools do not know about Cython and
check only for the availability of Pyrex.

See also: http://code.activestate.com/lists/python-distutils-sig/17586/