Bug 1700620 [wpt PR 28212] - Bump lxml from 4.6.2 to 4.6.3 in /css, a=testonly

Automatic update from web-platform-tests
Bump lxml from 4.6.2 to 4.6.3 in /css

Bumps [lxml](https://github.com/lxml/lxml) from 4.6.2 to 4.6.3.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](https://github.com/lxml/lxml/compare/lxml-4.6.2...lxml-4.6.3)

Signed-off-by: dependabot[bot] <support@github.com>
--

wpt-commits: a3750b96b9d651ef588a6a52daba19fac32c613a
wpt-pr: 28212

testing/web-platform/tests/css/requirements.txt

@@ -1,6 +1,6 @@
 Template-Python==0.1.post1
 html5lib==1.1
-lxml==4.6.2
+lxml==4.6.3
 mercurial==4.5
 six==1.15.0
 webencodings==0.5.1