Remove dependency on 'decorator'

I don't understand what this library does or why we needed it.
This commit replaces it with the good old functools.wraps(),
which fixes this crash that started occuring with
decorator 5.0.5:

```
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'
```

https://github.com/micheles/decorator/issues/109