ScriptNode binding : Release GIL when saving
ScriptNode binding : Release GIL when saving

This avoids a potential deadlock when the change to `ScriptNode::unsavedChangesPlug()` tries to cancel a background task which is partly implemented in Python, and is therefore waiting for the GIL.

ImageEngine/gaffer,ivanimanishi/gaffer,GafferHQ/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,ivanimanishi/gaffer,johnhaddon/gaffer,boberfly/gaffer,boberfly/gaffer,boberfly/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,hradec/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,hradec/gaffer,boberfly/gaffer,hradec/gaffer,andrewkaufman/gaffer,hradec/gaffer,hradec/gaffer,lucienfostier/gaffer