Loaders use it, but it is ignored by the pg storage.
However, as the Cassandra storage uses swh-model to validate
its input, it refuses this input from the loaders (and journal).

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.