Backend sub module

[frankenjoe]

Closes #170

Moves audbackend.Backend, audbackend.Artifactory, audbackend.FileSystem to sub-module audbackend.backend and renames Backend to Base.

For backward compatibility the old imports are still supported (tested in tests/test_legacy_import.py).

Documentation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d94b6e2) 100.0% compared to head (e3fb906) 100.0%.

Additional details and impacted files

Files	Coverage Δ
audbackend/__init__.py	100.0% <100.0%> (ø)
audbackend/backend/__init__.py	100.0% <100.0%> (ø)
audbackend/core/api.py	100.0% <100.0%> (ø)
audbackend/core/backend/artifactory.py	100.0% <100.0%> (ø)
audbackend/core/backend/base.py	100.0% <100.0%> (ø)
audbackend/core/backend/filesystem.py	100.0% <100.0%> (ø)
audbackend/core/conftest.py	100.0% <100.0%> (ø)
audbackend/core/interface/base.py	100.0% <100.0%> (ø)
audbackend/core/interface/versioned.py	100.0% <ø> (ø)

[hagenw]

Great, the separation is much nicer now between interface and backends.
It's also nice to have the backward compatibility with audbackend.Backend, but I guess the bigger chnage is anyway that audbackend.create() and audbackend.access() now return iinterfaces and not backends. But I guess it will be mainly the type hints that we need to fix in other packages.