Upgrade pillow from 7.2.0 to 8.1.2

[chigby]

Fixes a number of security vulnerabilities. See release notes for more details:

• https://pillow.readthedocs.io/en/stable/releasenotes/8.1.1.html#id1
• https://pillow.readthedocs.io/en/stable/releasenotes/8.1.2.html#id1