Drop Python 3.6 and 3.7 from trezorlib

[matejcik]

Keeping python 3.6 alive has proven borderline too complicated and it is way past EOL, so for trezorlib 0.14 I would drop compatibility for both 3.6 and 3.7 (also EOL).

This will allow us to use from __future__ import annotations which will finally bring in line the typing style in trezorlib and core code. I'm also guessing that there are some py3.6 leftovers in other parts of the code.