Reworked approach to #594. The cython_version variable is no longer in a Cython module; that was a non-starter since it meant a compiled module would have to be shipped with the source distribution in order for it to know what version of Cython the C files were generated with.  Now there's just a generated cython_version.py that gets touched up any time Cython is used to the build with a different version of Cython from the last time.  This file will ship with the source distribution and its presence will prevent Cython from being used for a release version. But for git clones of a release the cython_version.py *won't* be present and Cython will be used.  So this should cover all the relevant use cases.