[Reformat] Fix flake8 warnings

aleksis/core/checks.py

aleksis/core/forms.py

aleksis/core/mixins.py

aleksis/core/models.py

aleksis/core/registries.py

aleksis/core/search_indexes.py

aleksis/core/tasks.py

aleksis/core/templatetags/dashboard.py

aleksis/core/templatetags/data_helpers.py

aleksis/core/util/apps.py

aleksis/core/util/core_helpers.py

aleksis/core/util/messages.py

aleksis/core/util/notifications.py

aleksis/core/util/predicates.py

aleksis/core/util/sass_helpers.py

aleksis/core/util/search.py

aleksis/core/views.py