Update pytest-asyncio requirement from <=0.21.1 to <=0.23.5 in /backend

Updates the requirements on [pytest-asyncio](https://github.com/pytest-dev/pytest-asyncio) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-asyncio/releases)
- [Commits](pytest-dev/pytest-asyncio@v0.1.1...v0.23.5)

---
updated-dependencies:
- dependency-name: pytest-asyncio
  dependency-type: direct:development
...

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

backend/Pipfile

@@ -38,7 +38,7 @@ mypy = ">=1.3.1"
 pluggy = "<=1.4.0"
 pylint = ">=2.15.10"
 pytest = "<=8.0.2"
-pytest-asyncio = "<=0.21.1"
+pytest-asyncio = "<=0.23.5"
 pytest-cov = ">=4.0.0"
 pytest-xdist = ">=3.2.1"
 ruff = ">=0.0.292"