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>

requirements.txt

@@ -18,7 +18,7 @@ more-itertools==4.1.0
 numpy==1.14.2
 packaging==17.1
 pbr==4.0.1
-Pillow==5.0.0
+Pillow==8.1.1
 pluggy==0.6.0
 py==1.5.3
 pygame==1.9.3