Add "generator_stop" to default future imports for language_level=3. See #2580.
Add "generator_stop" to default future imports for language_level=3.
See #2580.

scoder/cython,scoder/cython,da-woods/cython,cython/cython,da-woods/cython,cython/cython,cython/cython,scoder/cython,da-woods/cython,cython/cython,da-woods/cython,scoder/cython