fix pytest dependency

Author: Richard Kuo (Onyx)

Diff for: backend/requirements/dev.txt

@@ -11,7 +11,7 @@ pandas==2.2.3
 posthog==3.7.4
 pre-commit==3.2.2
 pytest-asyncio==0.22.0
-pytest==7.4.4
+pytest==8.3.5
 reorder-python-imports-black==3.14.0
 ruff==0.0.286
 sentence-transformers==3.4.1