[Security] Bump urllib3 from 1.26.3 to 1.26.4

Bumps [urllib3](https://github.com/urllib3/urllib3) from 1.26.3 to 1.26.4. **This update includes a security fix.**
- [Release notes](https://github.com/urllib3/urllib3/releases)
- [Changelog](https://github.com/urllib3/urllib3/blob/main/CHANGES.rst)
- [Commits](urllib3/urllib3@1.26.3...1.26.4)

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

requirements.txt

@@ -35,7 +35,7 @@ transaction==3.0.1
 translationstring==1.4
 typing==3.7.4.3
 ujson==4.0.2
-urllib3==1.26.3
+urllib3==1.26.4
 venusian==3.0.0
 waitress==1.4.4
 WebOb==1.8.7