Importing Gtk will crash on the dynamic creation of Enum and Flag subclasses due to what looks to be an unsupported technique. The crash itself is happening because there is no NULL check after calling the C equivalent to "int.__new__". 

Stepping the code reveals python is trying to raise something like this:
TypeError: int.__new__(GtkSizeGroupMode) is not safe, use GtkSizeGroupMode.__new__()

In: <PythonSrc>/Object/typeobject.c : ~4848

The issue is Enum and Flag subclasses are setup in a tricky way and we cannot create the initial cached enum values using the dynamic subclasses standard creation method. Sudo code:

<static class>
class Enum(int):
    __new__ = int.__new__

<dynamic class>
class GtkSizeGroupMode(Enum):
    def __new__(cls, val):
        return self.__enum_values__[val]
GtkSizeGroupMode.__enum_values__ = [int.__new__(...), ...]

I have a fix for this but I would also like to include some refactoring to add clarity.