Fix the signature of unittest.TestCase.assertLogs #3513 (#3514)

stdlib/3/unittest/case.pyi

@@ -115,7 +115,7 @@ class TestCase:
                          expected_regex: Union[str, bytes, Pattern[str], Pattern[bytes]],
                          msg: Any = ...) -> _AssertWarnsContext: ...
     def assertLogs(
-        self, logger: Optional[logging.Logger] = ...,
+        self, logger: Optional[Union[str, logging.Logger]] = ...,
         level: Union[int, str, None] = ...
     ) -> _AssertLogsContext: ...
     @overload