[Security] Bump lxml from 3.7.1 to 4.6.3

Bumps [lxml](https://github.com/lxml/lxml) from 3.7.1 to 4.6.3. **This update includes security fixes.**
- [Release notes](https://github.com/lxml/lxml/releases)
- [Changelog](https://github.com/lxml/lxml/blob/master/CHANGES.txt)
- [Commits](lxml/lxml@lxml-3.7.1...lxml-4.6.3)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -26,7 +26,7 @@ http-crawler==0.1.2
     # via django-amber
 idna==2.8
     # via requests
-lxml==3.7.1
+lxml==4.6.3
     # via http-crawler
 markdown2==2.3.7
     # via django-markdown-deux