Bump pillow from 5.4.1 to 6.2.0

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.4.1 to 6.2.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](https://github.com/python-pillow/Pillow/compare/5.4.1...6.2.0)
Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -4,7 +4,7 @@ flask-socketio>=4.1.0
 python-socketio>=4.2.0
 flask_sockets>=0.2.1
 Werkzeug>=0.15.1
-Pillow==5.4.1
+Pillow==6.2.0
 requests>=2.20
 cffi>=1.12.3
 psutil>=5.2.0