Bump lxml from 3.7.3 to 4.6.3

Bumps [lxml](https://github.com/lxml/lxml) from 3.7.3 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/commits/lxml-4.6.3)

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

requirements-dev.txt

@@ -5,7 +5,7 @@ pycrypto==2.6.1
 webassets==0.12.1
 factory-boy==2.9.2
 pyquery==1.2.17
-lxml==3.7.3
+lxml==4.6.3
 cssselect==1.0.1
 coverage==4.4.1
 nose==1.3.7