lxml package needs to be updated to work with html_clean

[MaxFrank13]

Projects that use the edx-i18n-tools package are failing to install requirements. There was a recent minor version change to lxml that has projects using lxml.html.clean install the package with a setuptools dependency, like lxml[html_clean].

From the release notes: "The lxml.html.clean implementation suffered from several (only if used) security issues in the past and was now extracted into a separate library"

See the release notes for 5.2.