Django-heartbeat

Fix: compatibility Django4.0; change url import from django.urls

merge PR of Krzysztof Hoffmann <krzysiekplgmail.com>

handle warnnings; added docker test setup; update packages;

Fixes dependency to insecure psutil package

Add to the test matrix newer Django and Python versions

- Renamed "redis.py" checker to "redis_status.py".
- Renamed "memcached.py" checker to "memcached_status.py".
- Added heartbeat.checkers.host to the default checkers

\* The "redis_status" and "memcached_status" refactoring will impact your HEARTBEAT's checkers configuration.

Current valid checker names(for your settings.py - HEARTBEAT['checkers']):
- **heartbeat.checkers.build**
- **heartbeat.checkers.databses**
- **heartbeat.checkers.debug_mode**
- **heartbeat.checkers.distribution_list**
- **heartbeat.checkers.host**
- **heartbeat.checkers.memcached_status**
- **heartbeat.checkers.python**
- **heartbeat.checkers.redis_status**