PyGObject 3.10 Beta 2 Improves Compatibility with C89 Compilers

Thanks to Simon Feltman, Chun-wei Fan and Yanko Kaneti, PyGObject 3.10 Beta 2 fixes GObject signal methods, which now work with new annotations, fixes build issues on C89 Compilers and PEP-8 errors in propertyhelper.py, and uses G_IS_VALUE for checking return values in closure marshaling.