Remove version checking code, pyblake2 is installed by default on python 3.6+

This is a fix to workaround pip's inability to correctly solve
extra requirements (swh-model depends on swh-core[], but if other
packages depend on swh-model and swh-core[http], the 'http' extra
does not always get installed).

pgsql does not support it.

There is more repetition, but it's easier to read and
'%timeit Revision.from_dict(d)' is 5 times faster.

related to T1330

used by the swh-docs to generate images and schemas when building the
whole documentation.

to prevent sphinx from complaining about these not being included.

to the benefit of MultiHash api

Summary: to make sphinx happy.

Test Plan: make -C docs

Reviewers: ardumont

Reviewed By: ardumont

Differential Revision: https://forge.softwareheritage.org/D571

double back-quote (``) are not interpreted when inside a emphasis block.

This prevent from requiring a too much recent tox version to fire the
tests, especially this allows to run tests using stretch's tox version.

Promote "Resolution" to a new top-level section, because it's now rather
voluminous.

As part of this document the resolver limitations of identifiers.org, that get
in the way of PID contextual information.

Closes T1015
Closes T1223