[Cython] Python 3 wheels including bytecode files.

[Lisandro Dalcin]

After getting AppVeyor to work, I noticed that Python 3 wheels do
include __pycache__ directory. This also happens with in OS X as well
as Linux.

I think the root of the issue resides in setup.py, in the build_ext
command class workaround that seems to be a legacy thing from the time
Cython required 2to3. BTW, in setup.py, compile_cython_modules()
follows quite a different path in Python 2 versus 3.

Unless some of you believe we should keep things that way, I'll try to
fix setup.py to be as much independent as possible of the major Python
version.

-- 
Lisandro Dalcin
============
Research Scientist
Computer, Electrical and Mathematical Sciences & Engineering (CEMSE)
Numerical Porous Media Center (NumPor)
King Abdullah University of Science and Technology (KAUST)
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