when C compiling original Cython/Py2 sources in Py3, interpret unprefixed string literals as CPython's parser would

--HG--
rename : tests/run/generators.pyx => tests/run/generators_py.py

TST: numpy_test: run some of the buffer tests only when a sufficiently new Numpy version is available

Small fix was needed to make the buffer string parsing handle this case
correctly.

clean up special casing of non-portable builtin types, add option for disabling errors on unknown names