[Cython] Cython now runs on Py3

[Stefan Behnel]

Stefan Behnel wrote:
> Lisandro Dalcin wrote:
>> I'm thinking about an import hook to make runtests use it in Py3 ...
> 
> You could do what setup.py currently does: use distutils' build_py_2to3 to
> transform the sources so that they end up in the build/... directory (maybe
> you can retarget that), and then simply prepend that to sys.path before you
> import Cython. That also works incrementally, so it will only work on
> sources with modifications on later calls.

On second thought, maybe we should always install Cython to a local
directory as part of the test suite run. That would add compiling Cython to
the normal test run as well (which would prevent a couple of build
breakages that we had in the past).

Part of that would be to make Cython's build truly incremental (mainly the
compiler run), so that the test suite run doesn't unnecessarily loose time.

Stefan