Bump pillow from 4.1.1 to 6.2.0 (#291)

Bumps [pillow](https://github.com/python-pillow/Pillow) from 4.1.1 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](python-pillow/Pillow@4.1.1...6.2.0)

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

requirements.txt

@@ -1,6 +1,6 @@
 future==0.16.0
 lxml==3.7.3
-Pillow==4.1.1
+Pillow==6.2.0
 requests>=2.20.0
 six==1.10.0
 Werkzeug==0.15.3