bpo-41905: Add abc.update_abstractmethods() (GH-22485)

This function recomputes `cls.__abstractmethods__`.
Also update `@dataclass` to use it.

Lib/test/test_dataclasses.py

@@ -4,6 +4,7 @@
 
 from dataclasses import *
 
+import abc
 import pickle
 import inspect
 import builtins
@@ -3332,6 +3333,42 @@ class TestReplace(unittest.TestCase):
 
     ##     replace(c, x=5)
 
+class TestAbstract(unittest.TestCase):
+    def test_abc_implementation(self):
+        class Ordered(abc.ABC):
+            @abc.abstractmethod
+            def __lt__(self, other):
+                pass
+
+            @abc.abstractmethod
+            def __le__(self, other):
+                pass
+
+        @dataclass(order=True)
+        class Date(Ordered):
+            year: int
+            month: 'Month'
+            day: 'int'
+
+        self.assertFalse(inspect.isabstract(Date))
+        self.assertGreater(Date(2020,12,25), Date(2020,8,31))
+
+    def test_maintain_abc(self):
+        class A(abc.ABC):
+            @abc.abstractmethod
+            def foo(self):
+                pass
+
+        @dataclass
+        class Date(A):
+            year: int
+            month: 'Month'
+            day: 'int'
+
+        self.assertTrue(inspect.isabstract(Date))
+        msg = 'class Date with abstract method foo'
+        self.assertRaisesRegex(TypeError, msg, Date)
+
 
 if __name__ == '__main__':
     unittest.main()