Drop Python 2 support

[sblondon]

According to pypi page, the python2 support has been dropped with release 1.0.
The main branch in the repository is currently v.1.0.4 release (https://github.com/msgpack/msgpack-python/blob/main/msgpack/__init__.py#L9).

However, there is python2 specific code in the repository. For example:

• setup.py checks for python2
• msgpack/fallback.py could be simplified
• python2 is checked in tox.ini
• test/test_timestamp.py skip some tests with python2 (@pytest.mark.skipif(sys.version_info[0] == 2) ; test/test_extension.py has also a conditional

Are you interested by a PR to remove them?

[methane]

According to pypi page, the python2 support has been dropped with release 1.0.

Read it again.

[methane]

Are you interested by a PR to remove them?

pip dropped Python 2 support. So I will drop it too.
I don't have actual plan for msgpack 1.1 or 2.0. But PR to remove them is welcome.