Add test and changelog

Author: jacobtylerwalls

doc/whatsnew/fragments/9145.bugfix

@@ -0,0 +1,3 @@
+Restore "errors / warnings by module" section to report output (with `-ry`).
+
+Closes #9145

tests/test_self.py

@@ -981,6 +981,11 @@ def test_can_list_directories_without_dunder_init(tmp_path: Path) -> None:
             stderr=subprocess.PIPE,
         )
 
+    def test_warnings_by_module(self) -> None:
+        path = join(HERE, "regrtest_data", "unused_variable.py")
+        expected = "errors / warnings by module"
+        self._test_output([path, "-ry"], expected_output=expected)
+
     @pytest.mark.needs_two_cores
     def test_jobs_score(self) -> None:
         path = join(HERE, "regrtest_data", "unused_variable.py")