Resolve linter contradictions: consolidate config, fix isort first-party, pylint 10/10

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/1e4a1f06-55b9-4040-853e-6aaf9ee574c8

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/tests/test_dmarc_parser.py

@@ -2,6 +2,7 @@ import io
 import zipfile
 
 import pytest
+
 from app.services.dmarc_parser import DMARCParser
 from app.tests.test_data import SAMPLE_XML