Update pillow requirement from ==5.1.* to ==6.0.*

Updates the requirements on [pillow](https://github.com/python-pillow/Pillow) to permit the latest version.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@5.1.0...6.0.0)

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

requirements/test.txt

@@ -9,4 +9,4 @@ flake8==3.5.*
 pep8==1.7.*
 # for checking html
 beautifulsoup4==4.7.*
-Pillow==5.1.*
+Pillow==6.0.*