Breaking Change in v2.2.0: Python list does not provide a OneOf Template.

## Lists for OneOf Syntactic Sugar Gone

After upgrading to Confuse 2.2.0 lists in templates are no longer transformed into OneOf operators.

### Workarounds

- OneOf still works fine and can be substituted for the list shortcut.
- Reverting to Confuse 2.1.0 makes list shortcuts work again.

### Reproduction

Seen in my somewhat large repo: https://github.com/ajslater/comicbox
The confuse MappingTemplate is in
https://github.com/ajslater/comicbox/blob/v2.1.0/comicbox/config/__init__.py

### Resolution Suggestions

- Modify the docs and notify in release notes that the old syntactic sugar is gone.
- Or restore the functionality or if this was unintentional,

Start by reproducing the list-template behavior described in Confuse 2.2.0 and compare it with 2.1.0, using the MappingTemplate example in comicbox/config/__init__.py. Determine whether the removed list shortcut is intentional; done means either documenting the breaking change in the relevant docs and release notes or restoring the behavior, with regression coverage.

Written by the indexing model from the issue text.