⬆️ [bump][backend] Update pytest requirement 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@8.3.4...9.1.1)

---
updated-dependencies:
- dependency-name: pytest
  dependency-version: 9.1.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]

backend/pyproject.toml

@@ -30,7 +30,7 @@ documentation = "https://github.com/tomy0000000/kayman/wiki"
 dev = [
     "ipython>=9.4.0,<10",
     "mypy>=1.14.1,<1.18.0",
-    "pytest>=8.3.4,<9",
+    "pytest>=8.3.4,<10",
     "httpx>=0.28.1,<0.29",
     "factory-boy>=3.3.3,<3.4.0",
     "pre-commit>=4.3.0,<5",