Log Message:
py-cython: updated to 0.28.4

0.28.4:
Bugs fixed
* Reallowing tp_clear() in a subtype of an @no_gc_clear extension type generated \ 
an invalid C function call to the (non-existent) base type implementation.
* Exception catching based on a non-literal (runtime) tuple could fail to match \ 
the exception.
* Compile fix for CPython 3.7.0a2.