MutableMapping moved, update __init__.py

The attribute MutableMapping from the module collections got moved into collections.abc in python3.10.

If we don't make this update, users will see this error when they try to use this package: https://stackoverflow.com/questions/70943244/attributeerror-module-collections-has-no-attribute-mutablemapping.

Could this pull request be merged? It has been open over a year now. There is a user space hack but it is not very pretty.