bpo-43024: improve signature (in help, etc) for functions taking sent… (GH-24331) (GH-26773)

…inel defaults
(cherry picked from commit f73377d57c5272390de63cccc3c292c44689310a)

Co-authored-by: Irit Katriel <1055913+iritkatriel@users.noreply.github.com>
diff --git a/Lib/test/test_traceback.py b/Lib/test/test_traceback.py
index 5bd969d..dd94590 100644
--- a/Lib/test/test_traceback.py
+++ b/Lib/test/test_traceback.py
@@ -4,6 +4,7 @@
 from io import StringIO
 import linecache
 import sys
+import inspect
 import unittest
 import re
 from test import support
@@ -255,6 +256,21 @@ def test_exception_is_None(self):
         self.assertEqual(
             traceback.format_exception_only(None, None), [NONE_EXC_STRING])
 
+    def test_signatures(self):
+        self.assertEqual(
+            str(inspect.signature(traceback.print_exception)),
+            ('(exc, /, value=<implicit>, tb=<implicit>, '
+             'limit=None, file=None, chain=True)'))
+
+        self.assertEqual(
+            str(inspect.signature(traceback.format_exception)),
+            ('(exc, /, value=<implicit>, tb=<implicit>, limit=None, '
+             'chain=True)'))
+
+        self.assertEqual(
+            str(inspect.signature(traceback.format_exception_only)),
+            '(exc, /, value=<implicit>)')
+
 
 class TracebackFormatTests(unittest.TestCase):