Fix sorteddict stubs

Mypy now includes python/typeshed#6653
which triggered

```
stubs/sortedcontainers/sorteddict.pyi:88: error: Signature of "update" incompatible with supertype "MutableMapping"  [override]
stubs/sortedcontainers/sorteddict.pyi:88: note:      Superclass:
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, SupportsKeysAndGetItem[_KT, _VT], **kwargs: _VT) -> None
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, **kwargs: _VT) -> None
stubs/sortedcontainers/sorteddict.pyi:88: note:      Subclass:
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, Mapping[_KT, _VT], **kwargs: _VT) -> None
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None
stubs/sortedcontainers/sorteddict.pyi:88: note:          @overload
stubs/sortedcontainers/sorteddict.pyi:88: note:          def update(self, **kwargs: _VT) -> None
```

stubs/sortedcontainers/sorteddict.pyi

@@ -85,12 +85,19 @@ class SortedDict(Dict[_KT, _VT]):
     def popitem(self, index: int = ...) -> Tuple[_KT, _VT]: ...
     def peekitem(self, index: int = ...) -> Tuple[_KT, _VT]: ...
     def setdefault(self, key: _KT, default: Optional[_VT] = ...) -> _VT: ...
-    @overload
-    def update(self, __map: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
-    @overload
-    def update(self, __iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
-    @overload
-    def update(self, **kwargs: _VT) -> None: ...
+    # Mypy now reports the first overload as an error, because typeshed widened the type
+    # of `__map` to its internal `_typeshed.SupportsKeysAndGetItem` type in
+    #   https://github.com/python/typeshed/pull/6653
+    # Since sorteddicts don't change the signature of `update` from that of `dict`, we
+    # let the stubs for `update` inherit from the stubs for `dict`. (I suspect we could
+    # do the same for many othe methods.) We leave the stubs commented to better track
+    # how this file has evolved from the original stubs.
+    # @overload
+    # def update(self, __map: Mapping[_KT, _VT], **kwargs: _VT) -> None: ...
+    # @overload
+    # def update(self, __iterable: Iterable[Tuple[_KT, _VT]], **kwargs: _VT) -> None: ...
+    # @overload
+    # def update(self, **kwargs: _VT) -> None: ...
     def __reduce__(
         self,
     ) -> Tuple[