increasing the python requirement

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> Interestingly that thread also seems to recommend looking at PyPy.
> (Which just recently finally passed its benchmarks of being faster at
> compiling itself than cPython compiling PyPy.)

PyPy is also now faster at almost all Twisted benchmarks they've tried
too.  So definitely worth looking at I think.

-Andrew.