Improve type for setdefault()

- With one argument, it may return None
- With two arguments, it returns the default's type or the dict's value type.
- Also remove incorrect `= ...` from `pop()`. The one-argument case has its own overload.

Context: python/typing#1033 (reply in thread)

stdlib/typing.pyi

@@ -475,9 +475,12 @@ class MutableMapping(Mapping[_KT, _VT], Generic[_KT, _VT]):
     @overload
     def pop(self, __key: _KT) -> _VT: ...
     @overload
-    def pop(self, __key: _KT, __default: _VT | _T = ...) -> _VT | _T: ...
+    def pop(self, __key: _KT, __default: _VT | _T) -> _VT | _T: ...
     def popitem(self) -> tuple[_KT, _VT]: ...
-    def setdefault(self, __key: _KT, __default: _VT = ...) -> _VT: ...
+    @overload
+    def setdefault(self, __key: _KT) -> _VT | None: ...
+    @overload
+    def setdefault(self, __key: _KT, __default: _VT) -> _VT: ...
     # 'update' used to take a Union, but using overloading is better.
     # The second overloaded type here is a bit too general, because
     # Mapping[Tuple[_KT, _VT], W] is a subclass of Iterable[Tuple[_KT, _VT]],