Python 3.9 support

```
 /opt/hostedtoolcache/Python/3.8.7/x64/lib/python3.8/site-packages/chest/core.py:1: 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
151
    from collections import MutableMapping
```

Would you accept a PR fix, and make a new release?