Bump pillow from 7.1.0 to 8.1.1 in /requirements (#3373)

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)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/requirements.txt

@@ -23,7 +23,7 @@ pytz==2019.3
 python-magic==0.4.15
 unipath==1.1
 WeasyPrint==51
-Pillow==7.1.0
+Pillow==8.1.1
 gunicorn==19.9.0
 more-itertools==8.2.0
 pysolr==3.6.0