Bump pillow from 4.3.0 to 6.2.0

Bumps [pillow](https://github.com/python-pillow/Pillow) from 4.3.0 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@4.3.0...6.2.0)

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

requirements.txt

@@ -1,7 +1,7 @@
 six==1.11.0
 flask-diamond==0.5.2rc3
 PyPDF2==1.24
-Pillow==4.3.0
+Pillow==6.2.0
 Flask-Cache==0.13.1
 mdx-linkify==1.0
 mdx-journal>=0.1.4