Related to T2367

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

Also use direcly model object constructors instead of creating them from dict.

This also fixes the ci build [1]

[1] https://jenkins.softwareheritage.org/job/DVAU/job/tests/552/console

Required by swh-storage >= v0.0.172.

An ImportError was raised when one tries to start the vault server
using its CLI. Surely a side effect of recent changes in other swh
modules.

Related to D2400
Related to T2134

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

Related D2082

it has been around for months and is no longer advertized in --help string
since then

Similar to directory cooking (T1177), fetching a revision log must be performed
client-side in a paginated way to avoid storage timeouts and cooking errors.

Closes T1934

Related T1508

by describing the config file loading nightmare.

for the config_file argument, rather than declaration time.

This is required so that if this function is called with None as first
argument (which is what happens in cli.py when user do not specify a
config file option).

Long story short, this fixes the rpc-server cli command: if a config file
is not explicitely given as option, fall back to default config file search
behavior instead of crashing.

so that one should use:

  swh vault rpc-server

as in other swh packages.

The deprecated swh-vault command should still work as expected.

also set the main command name to 'vault' so the main 'swh' cli tool
can generate shell completion properly.

for the sake of consistency.

so tests are not broken when next swh.core version is published.

Might as well, as we already depend on 0.0.56+.