RPC server broken since python-decorator v5.0.5

ie. half the SWH stack

ERROR    testapp:app.py:1892 Exception on /foo [POST]                                    
Traceback (most recent call last):                                                       
  File "/home/dev/.local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()                                              
  File "/home/dev/.local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)                                                   
  File "/home/dev/.local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)                                                     
  File "/home/dev/.local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value                                                                          
  File "/home/dev/.local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()                                                         
  File "/home/dev/.local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)                           
  File "/home/dev/.local/lib/python3.7/site-packages/decorator.py", line 231, in fun     
    args, kw = fix(args, kw, sig)                                                        
  File "/home/dev/.local/lib/python3.7/site-packages/decorator.py", line 203, in fix     
    ba = sig.bind(*args, **kwargs)                                                       
  File "/usr/lib/python3.7/inspect.py", line 3015, in bind                               
    return args[0]._bind(args[1:], kwargs)                                               
  File "/usr/lib/python3.7/inspect.py", line 2930, in _bind                              
    raise TypeError(msg) from None                                                       
TypeError: missing a required argument: 'self'                                           

Reported to python-decorator here: https://github.com/micheles/decorator/issues/109

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Migrated from T3207 (view on Phabricator)