In Python 3.7, importing ABCs directly from the `collections` module shows a
warning (and in Python 3.8 it will stop working) - see
c66f9f8d39
This fixes various DeprecationWarnings such as those:
```
.../jinja2/utils.py:485: 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 MutableMapping
.../jinja2/runtime.py:318: 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 Mapping
```