[Cython] Pure python mode

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> Robert Kern wrote:
> Oh, never mind. I see what you're saying. But then why do we need
> cython.py if
> everything is provided by Cython/__init__.py?
>

At least for me, Python cannot import it by the name "cython" if only
Cython/... is available. And I think we want "import cython" (not "import
Cython") to work, as lowercase is more consistent with common Python
naming schemes in general and just "feels more right" for such core
language functionality.

(Also the module name would otherwise need to change in a lot of places --
when compiling under Cython, as opposed to running in Python, "cython" is
a magic, hard-wired module and "Cython" will not work, at least currently
(and my opinion is it never should))

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