collections.Mapping deprecated, to be removed in py3.9

[mattf]

Problem description

python -Wdefault -c 'from gensim.corpora import dictionary'

reports

gensim/corpora/dictionary.py:11: 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
  from collections import Mapping, defaultdict

expected no warning.

Versions

Linux-5.4.10-200.fc31.x86_64-x86_64-with-fedora-31-Thirty_One
Python 3.7.6 (default, Dec 19 2019, 22:52:49)
[GCC 9.2.1 20190827 (Red Hat 9.2.1-1)]
NumPy 1.18.1
SciPy 1.4.1
gensim 3.8.1
FAST_VERSION 1