Update adapters to use `collections.abc`?

[bannanc]

I'm getting this warning on my most recent travis tests

/home/travis/build/MobleyLab/chemper/chemper/chemper_utils.py:106: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

This is an easy fix and I'll get it into PR #82 when I get back to fixing that.

[bannanc]

Switching to just from collections import ... doesn't work in python 3.6 which we are still supporting. I recommend revisiting this issue after we add python 3.8 support.

[bannanc]

dask addresses this by adding its own compatibility file which imports in different ways allowing the same package to support multiple versions of python. This might be something to consider if it matters. For ChemPer collections is only used in adapters so the import could be

try:
    from collections.abc import ABC, abstractmethod, abstractclassmethod
except ImportError:
    from collections import ABC, abstractmethod, abstractclassmethod

I'm going to leave this up to whoever adds CI for python 3.8