Spurious `RemovedInDjango70Warning` for template string literals containing ".."

[Adam Johnson]

#35738 deprecated double-dot variable lookups in templates, but the warning emits a spurious RemovedInDjango70Warning for templates whose variable expression is a literal containing two consecutive dots, like:

{{ "hello..world" }}
{{ 'a..b'|upper }}
{{ _("a..b") }}

[Sarah Boyce <42296566+sarahboyce@…>]

In b5388a3:

Fixed #37257, Refs #35738 -- Prevented double-dot deprecation warnings for template literals.

The deprecation warning for double-dot variable lookups checked
str(filter_expression.var), which for constants is the resolved literal
value rather than a variable lookup. Templates containing string or translated
string literals with two consecutive dots, such as {{ "a..b" }} or
{{ 'a..b'|upper }}, therefore incorrectly raised a RemovedInDjango70Warning.

The check now only applies when the filter expression's variable is a
Variable lookup.

Regression in 5d911f2d2fecc703be91b2b9b28acc59d34b35f3.

[Sarah Boyce <42296566+sarahboyce@…>]

In 0060c6c:

[6.1.x] Fixed #37257, Refs #35738 -- Prevented double-dot deprecation warnings for template literals.

The deprecation warning for double-dot variable lookups checked
str(filter_expression.var), which for constants is the resolved literal
value rather than a variable lookup. Templates containing string or translated
string literals with two consecutive dots, such as {{ "a..b" }} or
{{ 'a..b'|upper }}, therefore incorrectly raised a RemovedInDjango70Warning.

The check now only applies when the filter expression's variable is a
Variable lookup.

Regression in 5d911f2d2fecc703be91b2b9b28acc59d34b35f3.

Backport of b5388a3a80cafcce2e34196d8e81cf5b48eb33bb from main.