Bump pillow from 4.0.0 to 8.2.0 in /requirements

Bumps [pillow](https://github.com/python-pillow/Pillow) from 4.0.0 to 8.2.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](python-pillow/Pillow@4.0.0...8.2.0)

---
updated-dependencies:
- dependency-name: pillow
  dependency-type: direct:production
...

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

requirements/base.txt

@@ -18,7 +18,7 @@ Markdown==3.2.1
 reportlab==3.5.6
 requests
 requests_oauthlib
-Pillow==4.0.0
+Pillow==8.2.0
 premailer
 populus
 psycopg2-binary==2.7.5