Related to T2367

- blackify all the python files,
- enable black in pre-commit,
- add a black tox environment.

Prior to this commit, the loader svn did an origin visit update to set the
snapshot. And then the loader core did the update to set the status.

This removes the origin visit update on the snapshot to let the loader core do
everything.

This actually migrates the loader svn to the latest swh.model api change.

This fixes the current ci build failure [1]

[1] https://jenkins.softwareheritage.org/job/DLDSVN/job/tests/540/

This was used in early versions when we had both a svn and a git-svn loader
implementation (for comparison purposes). The git-svn loader is long gone now
so might as well cleanup this as well.

Prior to this commit, the api change break the current loader implementation.
This fixes it by simply adapting the api calls. This will unbreak the current
ci build failure [1]

[1] https://jenkins.softwareheritage.org/job/DLDSVN/job/tests/536/console

Since the refactoring of swh.loader.core 0.0.74, so called BufferedLoaders don't
set the snapshot in the visit automatically. This loader would load data
properly, but fail to record the loaded snapshot into the visit.

swh-storage >= v0.0.174 requires swh-model objects in other proxies,
so the validation proxy must be before the other ones.

Required by swh-storage >= v0.0.172.

Related to T2188

That is:
- Push the retry behavior in configuration
- Migrate to the new BaseLoader class name
- Add the necessary conversion step for contents (for now)

Related to D2535

Related to T2134

Related to T2134

Related to T2134

With this commit, we stop passing parameters to the load method. We pass those
parameters to the loader's constructor.

Towards working on the cli unification

Related to T2134

Allows us to call tests on things other than python3, with the same settings.

Related D2082

Will be required by swh-loader-core v0.0.45.