gobjectmodule: do not clear floating flag in set_/get_property

Merged Mathieu Duponchelle requested to merge mathieudu/pygobject:property_fix into master

Setting a property installed in python from C fails when the object was created with g_object_new. This is a problem we're observing in gstreamer when inspecting elements implemented in python, gst-inspect will crash if the element has installed a property.

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