Bump urllib3 from 1.22 to 1.26.5 in /backend

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.22 to 1.26.5.
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.22...1.26.5)

---
updated-dependencies:
- dependency-name: urllib3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -71,7 +71,7 @@ text-to-image==0.0.5
 texttable==0.9.1
 unoconv==0.8.2
 uritemplate==3.0.0
-urllib3==1.22
+urllib3==1.26.5
 Wand==0.4.4
 websocket-client==0.48.0
 Werkzeug==0.14.1