Bump pillow from 5.4.1 to 9.3.0 in /docs (#455)

Bumps [pillow](https://github.com/python-pillow/Pillow) from 5.4.1 to 9.3.0.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/main/CHANGES.rst)
- [Commits](python-pillow/Pillow@5.4.1...9.3.0)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Henri Pesonen <henri.e.pesonen@gmail.com>

docs/requirements.txt

@@ -24,8 +24,8 @@ ipython==7.16.3
 ipython-genutils==0.2.0
 jedi==0.17.2
 MarkupSafe==2.0.1
+pillow==9.3.0
 future==0.18.3
-pillow==5.4.1
 alabaster==0.7.12
 commonmark==0.8.0
 nbsphinx