Errors in pypanda shutdown with multiple SIGINTs

[AndrewFasano]

If we go to terminate PyPANDA using ctrl-c (sending SIGINT), we'll get the following error. Should probably add a lock somewhere or a check to avoid starting our shutdown logic twice.

Exception ignored from cffi callback <function Panda._setup_internal_signal_handler.<locals>.SigHandler at 0x7efe6efb79d0>:                                                                                                        
Traceback (most recent call last):                                                                                                                                                                                                 
  File "/usr/local/lib/python3.8/dist-packages/pandare/panda.py", line 446, in SigHandler                                                                                                                                          
    self.end_analysis()                                                                                                                                                                                                            
  File "/usr/local/lib/python3.8/dist-packages/pandare/panda.py", line 535, in end_analysis                                                                                                                                        
    self.unload_plugins()                                                                                                                                                                                                          
  File "/usr/local/lib/python3.8/dist-packages/pandare/panda.py", line 733, in unload_plugins                                                                                                                                      
    self.pyplugins.unload_all()                                                                                                                                                                                                    
  File "/usr/local/lib/python3.8/dist-packages/pandare/pypluginmanager.py", line 264, in unload_all                                                                                                                                
    for name in self.plugins.keys():                                                                                                                                                                                               
RuntimeError: dictionary changed size during iteration