The addition of '-p no:pytest_swh_core' in pytest.ini is needed to
prevent pytest from loading the pytest_swh_core plugin which we do not
need here and which would require some more dependencies (e.g.
requests).

Related to T2610

Related to T2105

Related to T2105

Related to T2105

Related to T2367

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

It is now possible to install swh-model without dulwich (and Click, FWIW).

Users who want to use "swh identify" should "pip install swh-model[cli]".

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

to prevent confusion with setup()'s "extras_require" argument.

related to T1242

Summary:
We used to depend on the 'blake2s256' and 'blake2b512' names to be available in
hashlib.algorithms_available. It turns out that that's specific to OpenSSL 1.1.

We now try, in order:
 - blake2s256/blake2b512 as shipped by libssl1.1 (Python 3.5+ on Debian stretch
 and up)
 - blake2s/blake2b as built into Python 3.6+
 - pyblake2 if all else fails

While we're here, let's also avoid doing not-so-subtle hacks with hashlib
builtins.

Thanks to Alexios Zavras for the report.

Test Plan:
New unit tests added to check for behavior in all cases. Manually running the
tests on Python 3.4 + pyblake2, Python 3.5 and Python 3.6 as shipped by Debian
exercises all three cases.

Reviewers: zack, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D347

Currently only content and directory object types are supported, but more can
be added in the future.

Closes T1039

Summary: for those people with legacy openssls

Reviewers: anlambert, #reviewers!

Differential Revision: https://forge.softwareheritage.org/D224