[Cython] Cython master emits tons of 'foo' redeclared warnings

[Jeroen Demeyer]

When compiling SageMath with Cython master, I get literally thousands of 
warnings like

warning: sage/rings/polynomial/laurent_polynomial.pyx:177:10: 
'number_of_terms' redeclared

What is the meaning of this warning? As far as I know, Sage is not doing 
anything wrong here, so I feel like Cython is giving this warning 
erroneously.

I regularly compile Sage with Cython master and it is the first time 
that I am seeing this.