Revert "Use Extension from distutils instead of setuptools"

This reverts commit fc80d550c2971b57bd227147263c8dd9d330f8a2.
Distribute .c sources for cython files as fallback

Since Cython is a non-trivial dependency that may not be available on
every target system it is highly recommended to ship the generated .c
files along with the .pyx sources. See also:

http://docs.cython.org/src/reference/compilation.html#distributing-cython-modules
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.