Bump lxml from 3.6.1 to 4.6.3

Bumps [lxml](https://github.com/lxml/lxml) from 3.6.1 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-3.6.1...lxml-4.6.3)

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

Author: dependabot[bot]

requirements.txt

@@ -1,7 +1,7 @@
 beautifulsoup4==4.4.1
 bs4==0.0.1
 httplib2==0.19.0
-lxml==3.6.1
+lxml==4.6.3
 MarkupSafe==0.23
 nose==1.3.7
 numpy==1.11.0