gh-109848: Make test_rot13_func in test_codecs independent

[serhiy-storchaka]

• Issue: test_rot13_func in test_codecs is not independent #109848

[miss-islington]

Thanks @serhiy-storchaka for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-110504 is a backport of this pull request to the 3.12 branch.

[bedevere-app]

GH-110505 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Debian root 3.x has failed when building commit b987fdb.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/all/#builders/345/builds/6033) and take a look at the build logs.
4. Check if the failure is related to this commit (b987fdb) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/all/#builders/345/builds/6033

Failed tests:

• test_signal

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/root/buildarea/3.x.angelico-debian-amd64/build/Lib/test/test_signal.py", line 1287, in test_stress_delivery_dependent
    self.assertEqual(len(sigs), N, "Some signals were lost")
AssertionError: 7249 != 10000 : Some signals were lost