Prohibit naming a module cython.
Prohibit naming a module cython.

This causes the compiler to get confused and reuse the magic `cimport cython` module. See #2057.
scoder/cython,da-woods/cython,scoder/cython,da-woods/cython,scoder/cython,cython/cython,da-woods/cython,da-woods/cython,cython/cython,cython/cython,scoder/cython,cython/cython