Deprecation warnings when importing from collections

[IanRDavies]

There are a few places where importing from collections as opposed to collections.abc raises the following deprecation warning in Python3

DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.9 it will stop working

This can be fixed using the import style in json-spec/src/reference/util.py.

I will soon raise a PR using this import style everywhere.

[IanRDavies]

PR is ready to go!