Merge branch 'fix/lint-issues-in-6' into 'master'

aleksis/apps/untis/forms.py

aleksis/apps/untis/menus.py

aleksis/apps/untis/migrations/0001_initial.py

aleksis/apps/untis/model_extensions.py

aleksis/apps/untis/models.py

aleksis/apps/untis/preferences.py

aleksis/apps/untis/util/mysql/importers/absences.py

aleksis/apps/untis/util/mysql/importers/common_data.py

aleksis/apps/untis/util/mysql/importers/events.py

aleksis/apps/untis/util/mysql/importers/holidays.py

aleksis/apps/untis/util/mysql/importers/lessons.py

aleksis/apps/untis/util/mysql/importers/substitutions.py

aleksis/apps/untis/util/mysql/importers/terms.py

aleksis/apps/untis/util/mysql/main.py

aleksis/apps/untis/util/mysql/util.py

aleksis/apps/untis/util/xml/xml.py

aleksis/apps/untis/views.py