Version 0.4.0+ doesn't work on Python 3.6

[nosmoke]

You have added "from typing import Protocol" in the latest versions, and Protocol is not available in Python before version 3.8.
This is breaking some of our code that runs on Python 3.6 and until now assumed that as long as you are in version 0.* everything will be fine

[cgarciae]

Hey @nosmoke ! Thanks for reporting the issue. Fortunately this is a quite fix.

BTW: its a good practice to freeze dependencies (refer to exact versions) for production environments, check out tools like poetry or pipenv which helps you with this.

[cgarciae]

Fixed version 0.4.2 via #42