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
update setup.py script to use cythonize()

--HG--
extra : rebase_source : 6494c60e7f79a9cc85fe72a7adf325557f6de2be
prepare release of 0.19.1
fix optimisation of isinstance(X, type)
changelog
Merge branch 'master' into 0.19.x

Conflicts:
	CHANGES.rst
fix release date