Deprecated use of abstract base classes

[pnuu]

Code Sample, a minimal, complete, and verifiable piece of code

pytest trollsched/tests/test_satpass.py

Problem description

trollsched.utils has an import from collections import Mapping which won't work in Python 3.9.

Expected Output

No warnings.

Actual Result, Traceback if applicable

=============================== warnings summary ===============================
trollsched/utils.py:28
  /home/lahtinep/Software/pytroll/pytroll_packages/pytroll-schedule/trollsched/utils.py:28: 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

-- Docs: https://docs.pytest.org/en/stable/warnings.html
======================== 3 passed, 1 warning in 33.56s =========================

Versions of Python, package at hand and relevant dependencies

Python 3.8, pytroll-collectors 0.5.2 (the latest release).