Make sure that Callables are properly represented

See also comment of https://github.com/davidhalter/jedi/pull/1614#issuecomment-647054740

test/completion/pep0484_typing.py

@@ -417,6 +417,7 @@ type_in_out2()
 type_in_out2(float)
 
 def ma(a: typing.Callable[[str], TYPE_VARX]) -> typing.Callable[[str], TYPE_VARX]:
+    #? typing.Callable()
     return a
 
 def mf(s: str) -> int: