ScriptNode Binding : Fix GIL management bug.
ScriptNode Binding : Fix GIL management bug.

The binding for `ScriptNode::save()` was releasing the GIL and the registered `serialise` function was failing to re-acquire it. This could lead to SIGSEGV crashes if exceptions are thrown during serialisation.

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