Fix AttributeError in destructor
Sample error:
Error in atexit.run_exitfuncs:
Traceback (most recent call last):
File ".../python3.7/site-packages/pyjapc/japc.py", line 202, in __del__
if self._selector is None:
AttributeError: 'PyJapc' object has no attribute '_selector'
Since destructor is called with atexit
signal, given that the instance was garbage collected before the termination of the Python, this is a likely scenario.