bpo-29822: Make inspect.isabstract() work during __init_subclass__. (#678)

At the time when an abstract base class' __init_subclass__ runs,
ABCMeta.__new__ has not yet finished running, so in the presence of
__init_subclass__, inspect.isabstract() can no longer depend only on
TPFLAGS_IS_ABSTRACT.

Lib/test/test_inspect.py

@@ -229,6 +229,30 @@ class TestPredicates(IsTestBase):
         self.assertFalse(inspect.isabstract(int))
         self.assertFalse(inspect.isabstract(5))
 
+    def test_isabstract_during_init_subclass(self):
+        from abc import ABCMeta, abstractmethod
+        isabstract_checks = []
+        class AbstractChecker(metaclass=ABCMeta):
+            def __init_subclass__(cls):
+                isabstract_checks.append(inspect.isabstract(cls))
+        class AbstractClassExample(AbstractChecker):
+            @abstractmethod
+            def foo(self):
+                pass
+        class ClassExample(AbstractClassExample):
+            def foo(self):
+                pass
+        self.assertEqual(isabstract_checks, [True, False])
+
+        isabstract_checks.clear()
+        class AbstractChild(AbstractClassExample):
+            pass
+        class AbstractGrandchild(AbstractChild):
+            pass
+        class ConcreteGrandchild(ClassExample):
+            pass
+        self.assertEqual(isabstract_checks, [True, True, False])
+
 
 class TestInterpreterStack(IsTestBase):
     def __init__(self, *args, **kwargs):