Drop python2 support

[sblondon]

The PR removes python2 references and cases.
tox command is green on cpython3.9 (py39-c and py39-pure) on my local machine.

Close #518

[sblondon]

The last commit should fix the black step error in the Github CI (https://github.com/msgpack/msgpack-python/actions/runs/3630468029/jobs/6156022655).

However, when I run black . on my system, there are several other changes (a lot of a**b modified to a ** b). They are not added to the commit in order to limit the changes. I can add them if you want.

[ThomasWaldmann]

@sblondon it might depend on the black version used.

At @borgbackup, we have a version-fixed requirement in requirements.d/codestyle.txt so everybody uses exactly the same version.

At @msgpack there is also such a definition in requirements.txt, did you use it?

[sblondon]

At @msgpack there is also such a definition in requirements.txt, did you use it?

No, I runned the black which is installed on the system, not the version defined in requirements.txt file. So the differences generated by black are not useful here: all checks (including black) are green on Github CI.