WIP: Switch to new backend structure

[frankenjoe]

Apply necessary changes to audb in order to use the new backend structure that will be introduced with audackend>=1.0.0, see audeering/audbackend#76

[frankenjoe]

@hagenw

* It seems that actions are currently not run when setting dev as target branch.

• Building the docs and running the docstring examples are disabled until we have a published version of emodb with the new structure
• There are two more TODOs related to the tests, we need to fix

Otherwise transition seems straight-forward.

[codecov]

Codecov Report

Merging #305 (f946c47) into dev (3d1a20a) will decrease coverage by 1.7%.
The diff coverage is 75.8%.

Impacted Files	Coverage Δ
audb/core/conftest.py	0.0% <0.0%> (-100.0%)	⬇️
audb/core/api.py	100.0% <100.0%> (ø)
audb/core/load.py	100.0% <100.0%> (ø)
audb/core/load_to.py	100.0% <100.0%> (ø)
audb/core/publish.py	100.0% <100.0%> (ø)
audb/core/utils.py	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

[hagenw]

dev was not up-to-date with main. I updated it, but now you need to resolve a few conflicts.

[hagenw]

In audeering/audbackend#76 (comment) I proposed a migration plan that would allow us to also re-enable the emodb related tests here without affecting the tests of the main branch.

[frankenjoe]

Unfortunately, I was not able to properly rebase the branch. First it didn't let me push (due to some server errors of Github) and now it shows unexpected changes. I will close this PR and instead move the discussion to #306