Import collections ABCs from collections.abc, not collections

[sd2k]

This fixes a DeprecationWarning which would have broken in Python 3.8:

/home/ben/.virtualenvs/monitoring/lib/python3.7/site-packages/luigi/parameter.py:29
  /home/ben/.virtualenvs/monitoring/lib/python3.7/site-packages/luigi/parameter.py:29: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import OrderedDict, Mapping

/home/ben/.virtualenvs/monitoring/lib/python3.7/site-packages/luigi/scheduler.py:211
  /home/ben/.virtualenvs/monitoring/lib/python3.7/site-packages/luigi/scheduler.py:211: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    class OrderedSet(collections.MutableSet):

[dlstadther]

@sd2k We still support Python 2.7. To remove these warnings, we'll need to change the import based on the python version

Thanks for working to remove these warnings!

[sd2k]

@dlstadther Ah, of course! Thanks for checking. I've wrapped the imports in a try/except block now, and tests are passing.

[Tarrasch]

Thanks!!