[Cython] dynamically compile and import pure python modules

[mark florisson]

Hey,

For the fused type runtime dispatch I found it very convenient to use
the with statement, but that is not supported in Python 2.4. However,
the compiler could dynamically compile compiler code with the compiler
itself and import it (pyximport), if it is not needed to compile
Cython itself. I gave it a try and it seems to work like a charm (but
probably needs more testing :):
https://github.com/markflorisson88/cython/commit/0c2983056919f7f4d30a809724d7db0ace99d89b#diff-2

It seems like a bit of a hack, so I thought I'd bring it up here and
ask for opinions, i.e. ask if anyone thinks it is a terrible idea.

Mark