Bump pillow from 6.2.0 to 8.1.1

Bumps [pillow](https://github.com/python-pillow/Pillow) from 6.2.0 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@6.2.0...8.1.1)

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Author: dependabot[bot]