Bump lxml from 4.4.1 to 4.6.3 in /app

Bumps [lxml](https://github.com/lxml/lxml) from 4.4.1 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/compare/lxml-4.4.1...lxml-4.6.3)

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

app/requirements.txt

@@ -22,7 +22,7 @@ greenlet==0.4.15
 idna==2.8
 itsdangerous==1.1.0
 Jinja2==2.10.1
-lxml==4.4.1
+lxml==4.6.3
 MarkupSafe==1.1.1
 marshmallow==3.0.0
 mistune==0.8.4