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

Resolves #3207 (closed)

---

Migrated from D5421 (view on Phabricator)

swh/core/api/negotiation.py

@@ -24,10 +24,10 @@
 #
 
 from collections import defaultdict
+import functools
 from inspect import getcallargs
 from typing import Any, Callable, DefaultDict, List, NoReturn, Optional, Type
 
-from decorator import decorator
 from requests import Response
 
 
@@ -143,14 +143,15 @@ class Negotiator:
 def negotiate(
     negotiator_cls: Type[Negotiator], formatter_cls: Type[Formatter], *args, **kwargs
 ) -> Callable:
-    def _negotiate(f, *args, **kwargs):
-        return f.negotiator(*args, **kwargs)
-
-    def decorate(f):
+    def decorator(f):
         if not hasattr(f, "negotiator"):
             f.negotiator = negotiator_cls(f)
-
         f.negotiator.register_formatter(formatter_cls, *args, **kwargs)
-        return decorator(_negotiate, f)
 
-    return decorate
+        @functools.wraps(f)
+        def newf(*args, **kwargs):
+            return f.negotiator(*args, **kwargs)
+
+        return newf
+
+    return decorator