[3.13] gh-132762: Fix underallocation bug in dict.fromkeys()(gh-133627) (gh-133686)

The function `dict_set_fromkeys()` adds elements of a set to an existing
dictionary. The size of the expanded dictionary was estimated with
`PySet_GET_SIZE(iterable)`, which did not take into account the size of the
existing dictionary.
(cherry picked from commit 421ba589d0)

Co-authored-by: Angela Liss <59097311+angela-tarantula@users.noreply.github.com>

Lib/test/test_dict.py

@@ -312,17 +312,34 @@ class DictTest(unittest.TestCase):
         self.assertRaises(Exc, baddict2.fromkeys, [1])
 
         # test fast path for dictionary inputs
+        res = dict(zip(range(6), [0]*6))
         d = dict(zip(range(6), range(6)))
-        self.assertEqual(dict.fromkeys(d, 0), dict(zip(range(6), [0]*6)))
+        self.assertEqual(dict.fromkeys(d, 0), res)
+        # test fast path for set inputs
+        d = set(range(6))
+        self.assertEqual(dict.fromkeys(d, 0), res)
+        # test slow path for other iterable inputs
+        d = list(range(6))
+        self.assertEqual(dict.fromkeys(d, 0), res)
 
+        # test fast path when object's constructor returns large non-empty dict
         class baddict3(dict):
             def __new__(cls):
                 return d
-        d = {i : i for i in range(10)}
+        d = {i : i for i in range(1000)}
         res = d.copy()
         res.update(a=None, b=None, c=None)
         self.assertEqual(baddict3.fromkeys({"a", "b", "c"}), res)
 
+        # test slow path when object is a proper subclass of dict
+        class baddict4(dict):
+            def __init__(self):
+                dict.__init__(self, d)
+        d = {i : i for i in range(1000)}
+        res = d.copy()
+        res.update(a=None, b=None, c=None)
+        self.assertEqual(baddict4.fromkeys({"a", "b", "c"}), res)
+
     def test_copy(self):
         d = {1: 1, 2: 2, 3: 3}
         self.assertIsNot(d.copy(), d)