Add __contains__ to gdbm.gnu (#5511)

Fixes #5510

stdlib/dbm/gnu.pyi

@@ -17,6 +17,7 @@ class _gdbm:
     def __getitem__(self, item: _KeyType) -> bytes: ...
     def __setitem__(self, key: _KeyType, value: _ValueType) -> None: ...
     def __delitem__(self, key: _KeyType) -> None: ...
+    def __contains__(self, key: _KeyType) -> bool: ...
     def __len__(self) -> int: ...
     def __enter__(self) -> _gdbm: ...
     def __exit__(