Signature object provides methods like .bind(), which will be hard to define if a function has many signatures.  Also, inspect.signature currently returns one Signature object, that shouldn't be changed.

Wouldn't it be easier instead of this:
  type(obj)
  type(name, bases, mapping)

do this:
  type(obj_or_name, bases=None, mapping=None)

And explain what really is going on in the docstring?