cy exec test + correct building of locals dict in Cython frames for non-wide python 3 unicode builds

Choose PyString/PyBytesObjectPtr according to python version + support signals terminating the inferior.

Merged with libpython.py from Python3.2 + prevented continuation with negative pointer values gdb was giving to avoid segfaulting the inferior when using 'cy exec'

another fix for str() handling that works around internal use of EncodedString in StringNode, fix test for locals() in -3 mode

Disable some pyregr tests. As side effect this seems to solve ticket #537