use PyBytes_Type instead of PyString_Type in the generated sources, PyBytes fix for Py2.6, unicode fixes for star imports

'#define PyString_Type PyBytes_Type' in Py3: 'str' is considered a builtin and internally mapped to PyString_Type, which must also work in Py3

Casting a non-Python pointer type to a Python type no longer
generates an incref, unless one is required for other reasons.
    [Arc Riley]

invalidate the cache of all subtypes when updating a type's tp_dict (algorithm copied from typeobject.c in Py3)

--HG--
rename : tests/run/docstrings.pyx => tests/compile/docstrings.pyx