Drop support for end-of-life Python 3.6

[hukkin]

Describe the problem/need and solution

Python 3.6 is EOL. By dropping support we could do a few things:

• Reduce number of CI jobs
• Have nicer type annotations by using from __future__ import annotations (allows using built-ins like dict, list, tuple in annotations, rather than from typing import Dict, List, Tuple
• Consider if attrs dependency can be replaced with the Python 3.7 dataclasses stdlib (attrs is a superset so this may or may not be a good idea)

Guide for implementation

No response

Tasks and updates

No response

[chrisjsewell]

Consider if attrs dependency can be replaced with the Python 3.7 dataclasses stdlib (attrs is a superset so this may or may not be a good idea)

yeh thats exactly what I was gonna mention in #184 😄

[chrisjsewell]

Have nicer type annotations by using from __future__ import annotations

ah I hadn't actually realized this was possible in python 3.7 👍