🚨(pylint) make pylint work and fix issues found

Pylint was not installed and wrongly configured. After making
it work, we fix all the issues found so it can be added to our
CI requirements.

src/backend/pyproject.toml

@@ -65,6 +65,8 @@ dev = [
     "ipdb==0.13.13",
     "ipython==8.18.1",
     "pyfakefs==5.3.2",
+    "pylint-django==2.5.5",
+    "pylint==3.0.3",
     "pytest-cov==4.1.0",
     "pytest-django==4.7.0",
     "pytest==7.4.3",