PY-21224 Fixed: False positive unresolved base method when using six.with_metaclass() and multiple inheritance

Support metaclass and several base classes in six.with_metaclass

python/testSrc/com/jetbrains/python/codeInsight/PyClassMROTest.java

@@ -65,7 +65,11 @@ public class PyClassMROTest extends PyTestCase {
   }
 
   public void testSixWithMetaclass() {
-    assertMRO(getClass("C"), "B", "object");
+    assertMRO(getClass("C"), "B", "D", "object");
+  }
+
+  public void testSixWithMetaclassWithAs() {
+    assertMRO(getClass("C"), "B", "D", "object");
   }
 
   // PY-4183

python/testSrc/com/jetbrains/python/inspections/PyUnresolvedReferencesInspectionTest.java

@@ -734,6 +734,11 @@ public class PyUnresolvedReferencesInspectionTest extends PyInspectionTestCase {
     doMultiFileTest();
   }
 
+  // PY-21224
+  public void testSixWithMetaclass() {
+    doTest();
+  }
+
   @NotNull
   @Override
   protected Class<? extends PyInspection> getInspectionClass() {