Log Message:
py-cython: updated to 0.28.1

0.28.1:
Bugs fixed
PyFrozenSet_New() was accidentally used in PyPy where it is missing from the C-API.
Assignment between some C++ templated types were incorrectly rejected when the \ 
templates mix const with ctypedef.
Undeclared C++ no-args constructors in subclasses could make the compilation \ 
fail if the base class constructor was declared without nogil.
Bytes %-formatting inferred basestring (bytes or unicode) as result type in some \ 
cases where bytes would have been safe to infer.
None was accidentally disallowed as typed return value of dict.pop().