DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

When using libsass with python 3.7, one gets this warning message:

```
lib/python3.7/site-packages/sass.py:582: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  elif isinstance(custom_functions, (collections.Set, collections.Sequence)):
```