[3.6] bpo-29822: make inspect.isabstract() work during __init_subclass__ (#1979)

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.
(cherry picked from commit fcfe80ec25)

Lib/inspect.py

@@ -31,6 +31,7 @@ Here are some of the useful functions provided by this module:
 __author__ = ('Ka-Ping Yee <ping@lfw.org>',
               'Yury Selivanov <yselivanov@sprymix.com>')
 
+import abc
 import ast
 import dis
 import collections.abc
@@ -291,7 +292,27 @@ def isroutine(object):
 
 def isabstract(object):
     """Return true if the object is an abstract base class (ABC)."""
-    return bool(isinstance(object, type) and object.__flags__ & TPFLAGS_IS_ABSTRACT)
+    if not isinstance(object, type):
+        return False
+    if object.__flags__ & TPFLAGS_IS_ABSTRACT:
+        return True
+    if not issubclass(type(object), abc.ABCMeta):
+        return False
+    if hasattr(object, '__abstractmethods__'):
+        # It looks like ABCMeta.__new__ has finished running;
+        # TPFLAGS_IS_ABSTRACT should have been accurate.
+        return False
+    # It looks like ABCMeta.__new__ has not finished running yet; we're
+    # probably in __init_subclass__. We'll look for abstractmethods manually.
+    for name, value in object.__dict__.items():
+        if getattr(value, "__isabstractmethod__", False):
+            return True
+    for base in object.__bases__:
+        for name in getattr(base, "__abstractmethods__", ()):
+            value = getattr(object, name, None)
+            if getattr(value, "__isabstractmethod__", False):
+                return True
+    return False
 
 def getmembers(object, predicate=None):
     """Return all members of an object as (name, value) pairs sorted by name.