[issue46607] Add DeprecationWarning to configparser's LegacyInterpolation

[Hugo van Kemenade]

Hugo van Kemenade <hugovk+python at gmail.com> added the comment:

> > Other configparser deprecations were added in 3.2, but with DeprecationWarnings.

> Its deprecation was never documented anywhere in Doc/.

Correct, only in the docstring:

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

I've updated GH-30927 to say "deprecated in the docstring" instead of "deprecated in docs".

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