Fixed #37257 -- Prevented spurious double-dot deprecation warnings.

[adamchainz]

Trac ticket number

ticket-37257

Branch description

The deprecation warning for double-dot variable lookups checked str(filter_expression.var), which for constants is the resolved literal value rather than a lookup expression. Templates containing string, translated, or numeric literals with two consecutive dots, such as {{ "a..b" }} or {{ 'a..b'|upper }}, therefore emitted a spurious RemovedInDjango70Warning even though no variable lookup was involved, breaking test suites run with -W error and putting valid templates on track to become hard errors in Django 7.0. The check now only fires when the filter expression's variable is an actual Variable lookup.

Regression in 5d911f2.

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Claude 5 Fable did nearly all the work here, discovering the bug and writing the commit. I made minor edits. I agree with the report and approach.

Checklist

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• This PR does not disclose a security vulnerability (see vulnerability reporting).
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