bpo-46607: Add DeprecationWarning to LegacyInterpolation

[hugovk]

LegacyInterpolation has been deprecated in docs since its introduction in Python 3.2:

classLegacyInterpolation(Interpolation):
"""Deprecated interpolation used in old versions of ConfigParser. Use BasicInterpolation or ExtendedInterpolation instead."""

Let's add a DeprecationWarning for a couple of releases before removal.

https://bugs.python.org/issue46607

[gvanrossum]

Looks good. Let’s do this!

[tirkarthi]

@hugovk The PR introduces deprecation warnings in tests that need to be handled :

PYTHONWARNINGS=always ./python -Wall -m test test_configparser
0:00:00 load avg: 2.29 Run tests sequentially
0:00:00 load avg: 2.29 [1/1] test_configparser
/home/karthikeyan/stuff/python/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: 300 ms
Tests result: SUCCESS

[hugovk]

@tirkarthi Thanks, will check it!

[hugovk]

Please see #91480.