configparser: allow ConfigParser()[UNNAMED_SECTION] (#14759)

Author: trillllian

stdlib/configparser.pyi

@@ -258,9 +258,9 @@ class RawConfigParser(_Parser):
         ) -> None: ...
 
     def __len__(self) -> int: ...
-    def __getitem__(self, key: str) -> SectionProxy: ...
-    def __setitem__(self, key: str, value: _Section) -> None: ...
-    def __delitem__(self, key: str) -> None: ...
+    def __getitem__(self, key: _SectionName) -> SectionProxy: ...
+    def __setitem__(self, key: _SectionName, value: _Section) -> None: ...
+    def __delitem__(self, key: _SectionName) -> None: ...
     def __iter__(self) -> Iterator[str]: ...
     def __contains__(self, key: object) -> bool: ...
     def defaults(self) -> _Section: ...