Related to T2367

Summary:
Related to T1734

From abandonned D2799

Reviewers: ardumont

Reviewed By: ardumont

Differential Revision: https://forge.softwareheritage.org/D2974

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

This commit fixes the repository mapping to model. It broke when the listed
repository was either None or missing the id field [1]

[1] https://sentry.softwareheritage.org/share/issue/532d682182fc43d6a7a99400e3928811/

Related to T2188

Following the github's documentation [1]

[1] https://developer.github.com/v3/guides/best-practices-for-integrators/#dealing-with-abuse-rate-limits

Related to T2170

Prior to this commit, we sent the origin url as a versioned artifact.
Now we send the origin url as a CRAN's canonical one, and the associated list
of artifacts found there (only 1 today).

This decreases in smaller transaction which won't timeout

Related to T2160

Related to T2134
Related to D2409
Related to D2410

Related to T2134

The following commit adapts the return statements from both lister and their
associated tasks. This standardizes on what other modules (e.g. both dvcs and
package loaders) do.

This also adds tests to make sure that we properly send our version number to
upstreams.

And remove unnecessary fixture redefinition which causes indirection.

The scheduler module no longer initializes itself those task_type.

It's now defined in swh.scheduler

Otherwise, we are failing unicity constraint.

Related to T2070

Related bb5d405