Bump lxml from 4.6.4 to 4.9.1 in /backend

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.4 to 4.9.1. - [Release notes](https://github.com/lxml/lxml/releases) - [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt) - [Commits](lxml/lxml@lxml-4.6.4...lxml-4.9.1) --- updated-dependencies: - dependency-name: lxml dependency-type: direct:production ... Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]