[Cython] pyregr test suite

[mark florisson]

Hey,

Could we run the pyregr test suite manually instead of automatically?
It takes a lot of resources to build, and a single simple push to the
cython-devel branch results in the build slots being hogged for hours,
making the continuous development a lot less 'continuous'. We could
just decide to run the pyregr suite every so often, or whenever we
make an addition or change that could actually affect Python code (if
one updates a test then there is no use in running pyregr for
instance).

Mark