Bump pillow from 7.0.0 to 9.3.0 in /src/backend

Bumps [pillow](https://github.com/python-pillow/Pillow) from 7.0.0 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@7.0.0...9.3.0)

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

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Author: dependabot[bot]

src/backend/requirements.txt

@@ -12,7 +12,7 @@ jinja2==2.11.1
 markupsafe==1.1.1
 marshmallow-sqlalchemy==0.22.3
 marshmallow==3.5.1
-pillow==7.0.0
+pillow==9.3.0
 pyasn1==0.4.8
 pycparser==2.20
 python-jose[cryptography]==3.1.0