Bump lxml from 4.5.0 to 4.6.3 in /manifest-tools

Bumps [lxml](https://github.com/lxml/lxml) from 4.5.0 to 4.6.3.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.5.0...lxml-4.6.3)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

manifest-tools/requirements.txt

@@ -1,3 +1,3 @@
 dulwich == 0.19.15
-lxml == 4.5.0
+lxml == 4.6.3
 jira == 2.0.0