🚨(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/core/api/serializers.py

@@ -1,5 +1,4 @@
 """Client serializers for the People core app."""
-from drf_spectacular.utils import extend_schema_field
 from rest_framework import exceptions, serializers
 from timezone_field.rest_framework import TimeZoneSerializerField