DEP8 failure due to py3.10 incompatible import

From https://autopkgtest.ubuntu.com/results/autopkgtest-jammy/jammy/amd64/s/sepp/20220126_093801_68201@/log.gz:

autopkgtest [09:37:49]: test test-simple-sepp-call: [-----------------------
Traceback (most recent call last):
  File "/usr/bin/run_sepp.py", line 3, in <module>
    from sepp.exhaustive import ExhaustiveAlgorithm
  File "/usr/lib/python3/dist-packages/sepp/exhaustive.py", line 6, in <module>
    from sepp.algorithm import AbstractAlgorithm
  File "/usr/lib/python3/dist-packages/sepp/algorithm.py", line 11, in <module>
    from sepp.alignment import MutableAlignment
  File "/usr/lib/python3/dist-packages/sepp/alignment.py", line 29, in <module>
    from collections import Mapping
ImportError: cannot import name 'Mapping' from 'collections' (/usr/lib/python3.10/collections/__init__.py)

Upstream bug: https://github.com/smirarab/sepp/issues/117