Bump pillow from 7.1.0 to 8.1.1 in /docs

Bumps [pillow](https://github.com/python-pillow/Pillow) from 7.1.0 to 8.1.1.
- [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/7.1.0...8.1.1)

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

docs/requirements.txt

@@ -11,7 +11,7 @@ termcolor
 yacs
 tabulate
 cloudpickle
-Pillow==7.1.0
+Pillow==8.1.1
 future
 requests
 six