Bump pillow from 5.0.0 to 8.1.1

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.0.0 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](python-pillow/Pillow@5.0.0...8.1.1)

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

Author: dependabot[bot]

requirements.txt

@@ -6,7 +6,7 @@ GitPython==2.1.9
 numpy==1.14.3
 packaging==17.1
 pbr==4.0.3
-Pillow==5.0.0
+Pillow==8.1.1
 pytz==2018.3
 PyYAML==3.12
 pyzbar==0.1.7