Bump lxml from 4.3.4 to 4.4.0

Bumps [lxml](https://github.com/lxml/lxml) from 4.3.4 to 4.4.0.
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-4.3.4...lxml-4.4.0)

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

requirements.txt

@@ -1,6 +1,6 @@
 # native dependencies
 Pillow==6.1.0
-lxml==4.3.4
+lxml==4.4.0
 psycopg2==2.8.3
 Django==1.11.22