observer.py imports typing.Protocol which is not in Python 3.7

[yhay81]

I got an error when I run tox
E ImportError: cannot import name 'Protocol' from 'typing' (/Users/yhay81/.pyenv/versions/3.7.9/lib/python3.7/typing.py)

It is because observer.py import typing.Progocol and typing.Progocol is new in Python version 3.8.
https://docs.python.org/3/library/typing.html#typing.Protocol
(It is from #345)

I think this is one of a solution.

• Drop testing 3.7 in tox.
• Write notation comment in observer.py which says it is new feature of 3.8

I can make PR if this is okay.

[yhay81]

I dropped testing 3.7 in tox: #372
If it is okay that this repo do not support 3.7, I think this issue can be closed.

[faif]

Yeah, that's fine. In general it's good to keep up with the latest minor version + 1 or more minor older versions, but we don't need to support all of them.