fixed PY-8025 Enable rest, epytext and doctest highlighting in strings literal assigned to __doc__

python/testSrc/com/jetbrains/python/PythonHighlightingTest.java

@@ -170,6 +170,10 @@ public class PythonHighlightingTest extends PyTestCase {
     doTest(LanguageLevel.PYTHON32, true, false);
   }
 
+  public void testDocstring() {  // PY-8025
+    doTest(false, true);
+  }
+
   public void testYieldInNestedFunction() {
     // highlight func declaration first, lest we get an "Extra fragment highlighted" error.
     EditorColorsManager manager = EditorColorsManager.getInstance();