Support __class_getitem__ (PY-27913)

Update resolve for `[]` on definitions so `__class_getitem__` is supported.
Support this method (and `__mro_entries__`) in highlighting and completion.
Process it as a static method.

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

@@ -391,6 +391,11 @@ public class PythonHighlightingTest extends PyTestCase {
     doTest(LanguageLevel.PYTHON37, false, false);
   }
 
+  // PY-27913
+  public void testDunderClassGetItem() {
+    doTest(LanguageLevel.PYTHON37, false, true);
+  }
+
   @NotNull
   private static EditorColorsScheme createTemporaryColorScheme() {
     EditorColorsManager manager = EditorColorsManager.getInstance();