Since we use `REXML::Document` in the type signature for `#parse_xml`,
we can encounter an `uninitialized constant
Homebrew::Livecheck::Strategy::Xml::REXML` error in strategies like
`Sparkle` that use `Xml#parse_xml` internally when the Sorbet runtime
is used. Moving the related require outside of the `#parse_xml` method
and into the `Xml` strategy proper resolves this issue.
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