Fixed SOE in PyClass.isOverriddenMRO() check (PY-15002)

In Python 2 it is possible to create an endless metaclass chain by
defining '__metaclass__' at the top-level to be one of the classes
defined in the module. We assume that these circular metaclasses don't
override `mro()` in order to prevent infinite recursion in this check.

python/src/com/jetbrains/python/psi/impl/PyClassImpl.java

@@ -1359,6 +1359,9 @@ public class PyClassImpl extends PyBaseElementImpl<PyClassStub> implements PyCla
       final PyType metaClassType = cls.getMetaClassType(context);
       if (metaClassType instanceof PyClassType) {
         final PyClass metaClass = ((PyClassType)metaClassType).getPyClass();
+        if (cls == metaClass) {
+          return false;
+        }
         final PyFunction mroMethod = metaClass.findMethodByName(PyNames.MRO, true);
         if (mroMethod != null) {
           final PyClass mroClass = mroMethod.getContainingClass();

python/testData/inspections/PyUnresolvedReferencesInspection/incorrectFileLevelMetaclass.py

@@ -0,0 +1,11 @@
+class __metaclass__(object):
+    pass
+
+
+class C(object):
+    def foo(self):
+        pass
+
+
+c = C()
+c.foo()

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

@@ -460,6 +460,11 @@ public class PyUnresolvedReferencesInspectionTest extends PyInspectionTestCase {
     doTest();
   }
 
+  // PY-15002
+  public void testIncorrectFileLevelMetaclass() {
+    doTest();
+  }
+
   @NotNull
   @Override
   protected Class<? extends PyInspection> getInspectionClass() {