bpo-42749: Use dynamic version to test for unsupported bignum in Tk (GH-23966)

Tk can internally support bignum even if Tkinter is built without
support of bignum.
(cherry picked from commit 156b7f7052)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_tcl.py

@@ -446,7 +446,7 @@ class TclTest(unittest.TestCase):
             else:
                 self.assertEqual(result, str(i))
                 self.assertIsInstance(result, str)
-        if tcl_version < (8, 5):  # bignum was added in Tcl 8.5
+        if get_tk_patchlevel() < (8, 5):  # bignum was added in Tcl 8.5
             self.assertRaises(TclError, tcl.call, 'expr', str(2**1000))
 
     def test_passing_values(self):