devel/pygobject3-common: unbreak with DEFAULT_VERSIONS += python=3.9

pygobject-object.c:849:35: error: no member named 'tp_print' in '_typeobject'

offsetof(PyTypeObject, tp_print) };
^                      ~~~~~~~~

/usr/include/stddef.h:75:31: note: expanded from macro 'offsetof'
#define offsetof(type, field) __offsetof(type, field)

^                ~~~~~

/usr/include/sys/cdefs.h:480:34: note: expanded from macro 'offsetof'
#define offsetof(type, field) __builtin_offsetof(type, field)

^                        ~~~~~

pygobject-object.c:856:21: error: invalid application of 'sizeof' to an incomplete type 'int []'

for (i = 0; i < G_N_ELEMENTS(slot_offsets); ++i)
                ^~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/glib-2.0/glib/gmacros.h:822:36: note: expanded from macro 'G_N_ELEMENTS'
#define G_N_ELEMENTS(arr) (sizeof (arr) / sizeof ((arr)[0]))

^~~~~

Regressed by: https://github.com/python/cpython/commit/f9bab74d5b34
Obtained from: upstream