bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)

Special handling is needed, because for non-slots dataclasses the instance attributes are not set: reading from a field just references the class's attribute of the same name, which contains the default value. But this doesn't work for classes using __slots__: they don't read the class's attribute. So in that case (and that case only), initialize the instance attribute. Handle this for both normal defaults, and for fields using default_factory.
(cherry picked from commit d3062f672c)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>

Lib/test/test_dataclasses.py

@@ -2880,6 +2880,28 @@ def test_frozen_pickle(self):
                 self.assertIsNot(obj, p)
                 self.assertEqual(obj, p)
 
+    def test_slots_with_default_no_init(self):
+        # Originally reported in bpo-44649.
+        @dataclass(slots=True)
+        class A:
+            a: str
+            b: str = field(default='b', init=False)
+
+        obj = A("a")
+        self.assertEqual(obj.a, 'a')
+        self.assertEqual(obj.b, 'b')
+
+    def test_slots_with_default_factory_no_init(self):
+        # Originally reported in bpo-44649.
+        @dataclass(slots=True)
+        class A:
+            a: str
+            b: str = field(default_factory=lambda:'b', init=False)
+
+        obj = A("a")
+        self.assertEqual(obj.a, 'a')
+        self.assertEqual(obj.b, 'b')
+
 class TestDescriptors(unittest.TestCase):
     def test_set_name(self):
         # See bpo-33141.