minor code cleanups in metaclass utility code, fix compiler warning about unused function __Pyx_Py3MetaclassGet()

even when 'boundscheck' is disabled or indexing is trivial, call PyUnicode_READY() during unicode indexing to comply with C-API constraints

removed outdated dummy assignment code that originally prevented an unsed argument gcc warning for __pyx_self, fix one more case where __pyx_self needs to be marked as unused

--HG--
rename : tests/run/test_generator_frame_cycle.py => tests/run/generator_frame_cycle.py

fix CythonInclude after importlib changes, avoid searching for the built module, avoid changing sys.path

Conflicts:
	Cython/Compiler/ParseTreeTransforms.py