Bumps [pillow](https://github.com/python-pillow/Pillow) from 8.0.1 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/8.0.1...8.1.1

)

Multiple security issues in earlier versions, though generally not
exploitable through Odoo.

Backports are made in Debian/Ubuntu, but for pip users we should bump to
the latest "safe" version.

Courtesy of @dependabot

closes odoo/odoo#68169

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>