update python version testing

[Carreau]

3.3is EOL since last year, and pip print warnings in the test-suite.

[Carreau]

Note that if we drop 3.3 in testing we might want to explicitly remove 3.3 compatibility in requires_python and maybe remove some conditionals.

cc @mpacer

[mpacer]

rebased on master to resolve merge conflict

[Carreau]

Thanks !