Update pytest requirement from <9.0,>=8.3 to >=8.3,<10.0 in /backend

Updates the requirements on [pytest](https://github.com/pytest-dev/pytest) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest/releases)
- [Changelog](https://github.com/pytest-dev/pytest/blob/main/CHANGELOG.rst)
- [Commits](pytest-dev/pytest@8.3.0...9.0.2)

---
updated-dependencies:
- dependency-name: pytest
  dependency-version: 9.0.2
  dependency-type: direct:development
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -20,7 +20,7 @@ dependencies = [
 [project.optional-dependencies]
 dev = [
   "httpx>=0.28,<1.0",
-  "pytest>=8.3,<9.0",
+  "pytest>=8.3,<10.0",
   "ruff>=0.11,<1.0",
 ]