Bump lxml from 4.2.4 to 4.6.2 in /PPOCRLabel/requirements

Bumps [lxml](https://github.com/lxml/lxml) from 4.2.4 to 4.6.2.
- [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-4.2.4...lxml-4.6.2)
Signed-off-by: Ndependabot[bot] <support@github.com>

PPOCRLabel/requirements/requirements-linux-python3.txt

 pyqt5==5.10.1
-lxml==4.2.4
+lxml==4.6.2