Failure to load with Python 3.10

[davidharcombe]

Attempting to use messytables with Python 3.10 results in the following error:

  File "/layers/google.python.pip/pip/lib/python3.10/site-packages/messytables/core.py", line 2, in <module>
    from collections import Mapping
ImportError: cannot import name 'Mapping' from 'collections' (/opt/python3.10/lib/python3.10/collections/__init__.py)

This is due to Mapping moving to package collections.abc in Python 3.10.

core.py should be updated to take account of this.