Bump lxml from 3.4.0 to 4.6.3

Bumps [lxml](https://github.com/lxml/lxml) from 3.4.0 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-3.4.0...lxml-4.6.3)

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

requirements.txt

@@ -1,7 +1,7 @@
 beautifulsoup4==4.3.2
 coverage==3.7.1
 icalendar==3.8.3
-lxml==3.4.0
+lxml==4.6.3
 py==1.4.25
 pytz==2014.7
 tox==1.8.0