Bump pillow from 5.1.0 to 6.2.0 in /docs

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.1.0 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@5.1.0...6.2.0)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -114,7 +114,7 @@ pefile==2017.11.5
 pep8==1.7.1
 pickleshare==0.7.4
 pikopy==0.9.0
-Pillow==5.1.0
+Pillow==6.2.0
 Pint==0.8.1
 pkginfo==1.4.2
 plotly==3.4.0