Import abstract base classes from collections.abc

Import abstract base classes from collections.abc

In Python 3.7, importing ABCs directly from the `collections` module shows a
warning (and in Python 3.8 it will stop working) - see
https://github.com/python/cpython/commit/c66f9f8d3909f588c251957d499599a1680e2320

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
```