.c files only need to import psycopg.h: it will in turn import
dependencies from Python and libpq and configure.h. psycopg.h should be
the first to be imported, so the basic imports are not required in
the .h's
As a guideline I'm trying to import from the most specific to the most
generic to detect missing imports in the .h's.
* psycopg/cursor_type.c: add support for cyclic GC.
* psycopg/python.h: add definitions for Py_CLEAR() and Py_VISIT()
for compatibility with old versions of Python.
