Related T421

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

Related T703

- Add module docstring
- Add blake2s256 and blake2b512 in supported algorithms
- Spawn a new variable DEFAULT_ALGORITHMS as default computed
  algorithms for the main functions

Related T692

Remove the limit on the python3 version, this should be transparent.
If the hash requested is not available, this will raise with an
explanation on the error.

Related T692

The same caveat applies, will only be supported from python3.6 onward.

Related T692

The caveat is that it will only be supported when we will be using
python3 >= 3.5.

Related T692

Related T700

Since we compute it anyway, better return it along with the result