Dead code in URLValidator

[Mike Edmunds]

There's some dead code in django.core.validators.URLValidator which could be removed:

• The entire ​Trivial case failed. Try for possible IDN domain section is no longer useful. This code attempts to re-validate a failed URL after encoding an international domain name using IDNA 2003 ("punycode"). But the URLValidator regular expressions have allowed all IDNs since ​Commit 2e65d56 (for #20003, in 2015), so the super call will never fail with a validation error that switching to IDNA 2003 would let pass.

• The ​`ul` unicode letters property is no longer used. The regular expressions that had used it were moved into DomainNameValidator in ​Commit 4971a9a (for #18119, in Django 5.1).

For the first case, one way to verify the code is no longer in use is to run URLValidator on https://މިހާރު.com, which is a domain allowed by IDNA 2008 but prohibited by IDNA 2003. If the punycode() branch were coming into play, that URL would be rejected:

from django.core.validators import URLValidator
URLValidator()("https://މިހާރު.com")
# (No error)

from django.utils.encoding import punycode
punycode("މިހާރު.com")
# UnicodeError: Violation of BIDI requirement 3
# encoding with 'idna' codec failed

[Sarah Boyce <42296566+sarahboyce@…>]

In 9a891c38:

Refs #36007 -- Added IDNA 2008 test case for URLValidator.

Test a domain that is valid under IDNA 2008 but not IDNA 2003. This
helps verify that the branch in URLValidator which calls punycode() is
not actually being used for IDNs. punycode() implements IDNA 2003, so
the domain would fail to validate if that branch were active for IDNs.

[Sarah Boyce <42296566+sarahboyce@…>]

In 5405912:

Fixed #36007 -- Removed dead code from URLValidator.

The "Trivial case failed. Try for possible IDN domain" handling was
obsoleted by ticket-20003, which adjusted the regular expressions to
allow all international domain names (Refs #20003).

Uses of ul were moved to DomainNameValidator in ticket-18119
(Refs #18119).