Fix relative cimport for Cython package dirs

avoid "return"-ing from generator without NULL return value but no exception set (fails assertion in debug build)

work around crash in CPython's yield-from implementation for generators that "return" values using a StopIteration exception valu

add another fast-path in generator exit code that avoids exception normalisation of exact StopIteration objects

avoid setting explicit "None" as StopIteration exception value in generators and directly return NULL instead

disable unicode default encoding in pypy: PyBytes_AsStringAndSize() does not handle Unicode default encoding in pypy and _PyUnicode_AsDefaultEncodedString() does not return a cached borrowed reference but a new owned one