Drop support for old versions of Python

[SilkAndSlug]

Python 2.7 and 3.4 are EOL, and 3.5 and 3.6 are security-only (and 3.5 is no longer in Debian). I therefore propose to only support Python 3.7 and later, and to release this as EMSM v7. All against, say nay ...

[bryceharrington]

Fwiw, +1 sounds good to me. py2 is getting pretty obsolete by now.