avoid modifying an iterable while iterating in gstlal_ll_inspiral_event_plotter
I was seeing errors like:
Traceback (most recent call last):
File "/usr/bin/gstlal_ll_inspiral_event_plotter", line 488, in <module>
event_plotter.start()
File "/usr/lib64/python3.6/site-packages/gstlal/events.py", line 119, in start
self.process()
File "/usr/lib64/python3.6/site-packages/gstlal/events.py", line 108, in process
self.handle()
File "/usr/bin/gstlal_ll_inspiral_event_plotter", line 236, in handle
for event_key, event in self.events.items():
RuntimeError: OrderedDict mutated during iteration
in the error log for gstlal_ll_inspiral_event_plotter jobs.