bump minimum python version to 3.6

Summary:
Version 3.5 has reached EOL in september 2020, and pip has dropped
support for it in version 21.0 (january 2021).

One of the benefits of dropping support for 3.5 is that it will allow us
to use modern syntax for python tyephints for attributes
(https://www.python.org/dev/peps/pep-0526/)

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9270