* Upgrade django and stuff
* Update to Django 2.2 and recent versions of similar packages
* Provide explicit orderings to all models used in paginated queries
* Resolve naive datetime warnings in test suite
* Deal with deprecation warnings
* Fix sqlparse version
* add basic instrumentation possibilities to pretix
* make tabs to spaces
* apply flake8
* implement upstreams suggestions, round 1
* adjust naming of redis-connection
* address noredis
* add view for metrics
* implement HTTP basic auth in front of metrics-endpoint
* rename labelset
* make flake8-clean
* implement upstreams suggestions, round 2
* correct minor slipups
* fix missing return
* let isort add an empty line
* implement test for counter
* implement upstream suggestions, round 3
* correct typo
* implement first test for view
* finish view-test
* fix deprecated keyword
* implement upstream-suggestions, round 4
* implement test for gauge
* test exceptions as well
* add db-decorator