Fix import error on pypy.

This patch fix the only import error left when running the cffi branch on pypy (except the vector type one in `array.py` and I'm waiting for pypy 2.3 to see if there's any difference on that.)

The code is basically copied from the master branch with minor white space clean up.

Unfortunately my GPU opencl driver is broken (beignet on i7-4702HQ) so I cannot run the tests yet...