cythonize 0.29.x: use force=None as default instead of force=False

[haampie]

I completely overlooked this in #5307,
that backport was useless because the signature of cythonize is

def cythonize(..., force=False, ...)

in 0.29.x instead of

def cytonize(..., force=None, ...)

like it is on 3.x, meaning that setting CYTHON_FORCE_REGEN=1 had no effect.

[haampie]

@scoder does cython still do 0.29 releases? If so, this would be helpful to have

[scoder]

Right, that should work "better". :)
Thanks.