PyGObject 3.8.1 Brings GStreamer Rules

PyGObject 3.8.1 brings GStreamer rules, implemented in the pygi-convert.sh file, as well as a TreeModelFlags rule, implemented in pygi-convert. Moreover, the enum and flags marshaling type assumptions were fixed in this stasble release of PyGObject. The stack corruption, which appeared due to incorrect format for argument parser, was fixed, and the g_strdup() function is now used consistently.