Onionperf errors when no subcommand is given (python3)

Not a major issue by any means, but OP errors when ran without any arguments. This seems to be a behaviour change from Python2 to Python3. The expected behaviour was previously to print help.

~/onionperf$ onionperf
Traceback (most recent call last):
  File "/usr/local/bin/onionperf", line 532, in <module>
    if __name__ == '__main__': sys.exit(main())
  File "/usr/local/bin/onionperf", line 351, in main
    args.func(args)
AttributeError: 'Namespace' object has no attribute 'func'

I've attached a patch to get back that behaviour. See also https://bugs.python.org/issue16308.

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