Bump pillow from 7.1.0 to 8.1.1

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](python-pillow/Pillow@7.1.0...8.1.1)

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

requirements.txt

@@ -6,4 +6,4 @@ Django==3.1.4
 easy_thumbnails==2.7.1
 future==0.18.2
 Markdown==2.6.11
-Pillow==7.1.0
+Pillow==8.1.1