extract normal cythonize script test from package compilation test as the latter is not supported in Py3.[012]

--HG--
rename : tests/build/cythonize_script.srctree => tests/build/cythonize_script_package.srctree

move cythonize script to Cython.Build.Cythonize to make it generally available and keep bin/cythonize only as a pure start script

Also added note to change log.

allow srctree header commands to be wrapped in triple string quotes to stop IDEs from complaining about them

fix "exclude_failures" mode in cythonize() by also ignoring stripped C files from compilation failures