DumbNotifiedProperty fails when used with different objects

[eoinell]

Running https://github.com/nanophotonics/nplab/blob/python3/nplab/instrument/spectrometer/__init__.py , the variable_int_enabled and filename DumbNotifiedProperties fail to callback their registered functions. [python3].
Notified properties seem to work fine however, and notified_property's docstring works fine with but dumb and ordinary notified properties.

[eoinell]

I traced why the registered functions weren't being called, the WeakSet holding the callbacks was being garbage-collected. Replacing this with set fixes this issue, at the potential cost of some memory.