T423 explicit execption chaining

move 'with' statement implementation back into WithTransform to fix 'with' statement usage for compiler directives

--HG--
rename : tests/run/withstat.pyx => tests/run/withstat_py.py

BuildExecutable: use correct executable extension, build dynamically instead of statically when possible

link statically against libpython in BuildExecutable (hack to make it work if libpython is in a non-standard directory)

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