Fix DeprecationWarning in Python 3.7

[stevenwinfield]

...while preserving backward-compatibility with previous versions of Python.

The warning message is:

Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
class _SelectorMapping(collections.Mapping):

[harvimt]

I fixed this slightly differently in #111 (since I'm dropping support for old pythons anyway)