Issue with importing typing library in Python 3.7

[jmettes]

Issue Description

I'm trying to run puresnmp in AWS Lambda (Greengrass). I ran into this issue:

type object 'Callable' has no attribute '_abc_registry'

More info on typing library issue: python/typing#573

I think this could be fixed by changing the dependency on typing to <= 3.5

[exhuma]

This is already the case so I'm not sure why you're getting this.

What version of puresnmp and Python are your currently using (including the minor number).