Issue #, if available:

Partly fixes #129 by removing most obvious problem (my training script ran afterwards, but this is no guarantee).

Description of changes:

Since Python 3.3 (over 10 years ago), the following is deprecated: from collections import Mapping. In Python 3.9:

<stdin>:1: DeprecationWarning: Using or importing the ABCs
  from 'collections' instead of from 'collections.abc' is deprecated
  since Python 3.3, and in 3.10 it will stop working

This PR replaces the offending statement as advised in the warning message.

Testing done:

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