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

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extra : rebase_source : bafa0a661abe97868fe254b3ce2b5e63b8654990
update setup.py script to use cythonize()

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extra : rebase_source : 6494c60e7f79a9cc85fe72a7adf325557f6de2be