Fix collections deprecation warning

[chrisjsewell]

Fixes:

scss/types.py:6: 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 Iterable

[coveralls]

Coverage decreased (-0.01%) to 66.801% when pulling 147a490 on chrisjsewell:fix-deprecate/collections into 6bf001e on Kronuz:master.

[amotoki]

LGTM as pyScss supports python 2.7 too.

/cc @amotoki

[loic-simon]

This warning will become an error in Python 3.10, released in a few days.

Is it possible to merge this fix asap, since it use a try/except block (so no issue with 2.7) ?

/cc @e0ne

Thanks

[GammaGames]

Would be cool if this were merged and released. Can't use the library in 3.10.

[wynnw]

This hit us too. Guess we'll have to run a fork now if the project isn't putting out anymore easy updates. Looks pretty dead. (Update - python-libsass is a good alternative that doesn't have this problem)

[manchandavishal]

LGTM,
@e0ne , Could you please approve this asap and release it.
Thanks.

[mauritsvanrees]

Fix was included in 1.4.0, released today. I don't see warning messages anymore at startup. Thanks Ivan!