Related to T2367

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

- add mention to the SANER 2020 paper, as a more extensive reference of the
  whole process
- point to the `swh graph compress` CLI as compression process driver
- uniform sectioning
- reference data model and PID documents via internal IDs rather than absolute
  URLs to docs.s.o

Related to T2188

Multiple identifiers (of either kind) can be passed either directly on the CLI
or via stdin. In the latter case logical lines in stidn will be preserved in
stdout.

Closes T2112

... instead of only showing *responses*, without matching requests

in addition to /

as /walk is temporarily disabled for now

works for both random and non-random walks

Closes T2084

we need to fix T1969 before opening this up, and in most cases what one wants
is really /randomwalk anyway

Closes T1888

it is the only sane way to have decent semantics in presence of edge
restrictions, without having to visit twice all successors