Fix CI configuration: add pytest installation

.gitlab-ci.yml

@@ -13,13 +13,15 @@ services:
 
 before_script:
   - python -V
+  - python -m pip install --upgrade pip
+  - pip install pytest
   - pip install -r backend/requirements.txt
 
 test:
   stage: test
   script:
     - cd backend
-    - pytest -v
+    - python -m pytest -v
   only:
     - main
     - Testing