[Cython] Cython 0.26 release

[Jeroen Demeyer]

Could the error from
https://github.com/cython/cython/issues/1732
at
https://github.com/cython/cython/blob/master/Cython/Compiler/Symtab.py#L2099
be a warning instead?

SageMath has quite a bit of code hitting this error. It would make our 
life easier if we could fix it gradually instead of being forced to fix 
it all before upgrading to Cython 0.26.