Python: Add python 3.10 support and remove deprecated versions

Describe your change

I've added Python 3.10 to your tox configuration to check that the library is compatible with this version. It's working well on my machine.

I've also removed the stale version of Python 3 which are actually deprecated[^1].

Release	Released	Security Support
3.6	22 Dec 2016	Ended 9 months ago (23 Dec 2021)
3.5	12 Sep 2015	Ended 2 years ago (13 Sep 2020)
3.4	15 Mar 2014	Ended 3 years and 6 months ago (18 Mar 2019)
3.3	29 Sep 2012	Ended 5 years ago (29 Sep 2017)

In a second commit, I've removed support for Python 2.7 and upgrade the codebase to Python 3.6[^2]

Better review commit by commit.
I can totally drop the second commit that removes the support of Python 2.7 so let me know.

[^1]: Table that describe python end of life: https://endoflife.date/python [^2]: I've used python-upgrade for this: https://github.com/asottile/pyupgrade#readme