Cleanup

[rossnomann]

• Fixed coding-style issues (flake8, isort, mypy)
• Changed deprecated pytest.yield_fixture to pytest.fixture
• Fixed wrong tree_id when rebuilding subtree in MPTT mixin
• Fixed some tests (still need to enable tests for users app)
• Removed Travis CI config and added a github workflow
• Added missing packages to Pipfile and setup.py
• Specified package versions in Pipfile and setup.py
• Replaced aioredis by redis-py and prepared method_redis_once for deprecation/removal

Looks like it works on python versions from 3.7 to 3.9 For 3.10 we need to upgrade third-party packages such as aioworkers.