collections.abc in python2

[ftherrien]

A lot of us are still using python 2 (even though life is short...) and abc is still in collections for that version of the standard library such that import pylada fails with:

from collections.abc import MutableSequence
ImportError: No module named abc

This is after 09ce322. Maybe do something like:

try:
    from collections.abc import MutableSequence
except ImportError:
    from collections import MutableSequence

That would get rid of the deprecation warning.

[ftherrien]

We are having the same problem with getfullargspec and args.varkw from e56fd4b