src/_image.cpp: Fix memory leak

- Objects created with PyInt_FromLong() must be decrefed.
Prepare to reenable Python3 by using macros

- mplutil.h: Add macros to unify the Python2 and Python3 APIs.
- _image.cpp: First use of new macros.
- _ft2font.cpp: Fix memory leaks by using new macros
                setIfL(), setLfL() and setLfUL().