Make self a positional-only argument in all update overloads

Before, overriding this function without the type-checker complaining was impossible, because the overloads hat different requirements for self (positional-only vs. positional or keyword) that could not be fulfilled by a single function implementation.

Fixes #15310

Author: iFreilicht

stdlib/typing.pyi

@@ -840,7 +840,7 @@ class MutableMapping(Mapping[_KT, _VT]):
     @overload
     def update(self: SupportsGetItem[str, _VT], m: Iterable[tuple[str, _VT]], /, **kwargs: _VT) -> None: ...
     @overload
-    def update(self: SupportsGetItem[str, _VT], **kwargs: _VT) -> None: ...
+    def update(self: SupportsGetItem[str, _VT], /, **kwargs: _VT) -> None: ...
 
 Text = str