make SwitchTransform always generate a 'default' clause to silence C compiler warnings about unmatched enum values

simplify search for duplicate values in SwitchTransform and make it safer for unexpected value nodes

Conflicts:
	CHANGES.rst

int -> Py_ssize_t in cpython pxd files

--HG--
extra : rebase_source : 6494c60e7f79a9cc85fe72a7adf325557f6de2be

remove 'from datetime cimport *' line from cpython/__init__.pxd as it refers to a separate module, not the core C-API

--HG--
extra : rebase_source : bafa0a661abe97868fe254b3ce2b5e63b8654990

remove 'from datetime cimport *' line from cpython/__init__.pxd as it refers to a separate module, not the core C-API

--HG--
extra : rebase_source : bafa0a661abe97868fe254b3ce2b5e63b8654990

--HG--
extra : rebase_source : 6494c60e7f79a9cc85fe72a7adf325557f6de2be

This reverts commit 12d7dbb3.