gh-90765: Catch LegacyInterpolation deprecation warning

[hugovk]

Follow on from gh-90765, see #30927 (comment).

Before

$ PYTHONWARNINGS=always ./python.exe -Wall -m test test_configparser
Raised RLIMIT_NOFILE: 256 -> 1024
0:00:00 load avg: 5.65 Run tests sequentially
0:00:00 load avg: 5.65 [1/1] test_configparser
/Users/huvankem/github/cpython/Lib/test/test_configparser.py:1031: DeprecationWarning: LegacyInterpolation has been deprecated since Python 3.2 and will be removed from the configparser module in Python 3.13. Use BasicInterpolation or ExtendedInterpolation instead.
  interpolation = configparser.LegacyInterpolation()

== Tests result: SUCCESS ==

1 test OK.

Total duration: 476 ms
Tests result: SUCCESS

After

$ PYTHONWARNINGS=always ./python.exe -Wall -m test test_configparser
Raised RLIMIT_NOFILE: 256 -> 1024
0:00:00 load avg: 5.67 Run tests sequentially
0:00:00 load avg: 5.67 [1/1] test_configparser

== Tests result: SUCCESS ==

1 test OK.

Total duration: 468 ms
Tests result: SUCCESS

[ghost]

All commit authors signed the Contributor License Agreement.

[hugovk]

@tirkarthi Does this need a news file or skip-news label?

[tirkarthi]

LGTM. Thanks @hugovk .

[AlexWaygood]

All commit authors signed the Contributor License Agreement.

@ambv any idea why the CLA CI check is red, even though the comment here says it's green?

[hugovk]

I have already signed the old CLA. When I first created the PR, the CLA comment was red:

So I clicked the link to sign the new one anyway. It turned the comment green but not the check: