Related to T2105

This allows for a more in-depth refactor of the logic in each view. We
can create views that simply override a specific behavior of some
generic views, instead of having to write ugly spaghetti code in the
form of nested functions that return other parametrized functions.

We used to have PROT_READ | PROT_WRITE and MAP_PRIVATE for readonly
maps. However, this would reserve copy-on-write space and make the
service OOM. We could also use MAP_NORESERVE to avoid committing for
that much memory in advance, but we really don't need to have writable
space at all, so we just set the prot flags properly.

Fix T2492

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