PR: Fix "from collections import Mapping, MutableMapping" with Python 3.8.

[vidakDK]

In Python 3.8 from collections import Mutable no longer works, instead
from collections.abc import Mutable must be used

[vidakDK]

Ah this needs to work in Python 2 as well? Not sure if worth to fix if Python 2 will be gone in a couple months :)

[KelSolaar]

Hi @vidakDK,

I'm not sure yet how long after Python 2 death we will maintain Colour compatibility with it but it will not go instantly.

What about using something like that instead:

import sys

if sys.version_info[:2] >= (3, 8):
    from collections.abc import Mapping, MutableMapping
else:
    from collections import Mapping, MutableMapping

[KelSolaar]

Looking at this one and seems like there are a few more imports to correct:

/colour-science/colour/colour/algebra
    interpolation.py
        from collections import OrderedDict, Mapping
/colour-science/colour/colour/continuous
    multi_signals.py
        from collections import Iterator, Mapping, OrderedDict, Sequence
    signal.py
        from collections import Iterator, Mapping, OrderedDict, Sequence
/colour-science/colour/colour/plotting
    models.py
        from collections import Mapping
/colour-science/colour/colour/utilities
    array.py
        from collections import Mapping
    data_structures.py
        from collections import Mapping, MutableMapping

Are you happy to do that @vidakDK, I can do it otherwise?

[KelSolaar]

I was hoping that six would help here but seeing that this PR does not move, I don't think it will: benjaminp/six#241

[KelSolaar]

Closing this one for Hacktoberfest purposes, thanks @vidakDK!