Voluptuous

**Changes**:
- [435](https://github.com/alecthomas/voluptuous/pull/435): Extended a few tests (`Required` and `In`)
- [425](https://github.com/alecthomas/voluptuous/pull/425): Improve error message for `In` and `NotIn`
- [436](https://github.com/alecthomas/voluptuous/pull/436): Add sorted() for `In` and `NotIn` + fix tests
- [437](https://github.com/alecthomas/voluptuous/pull/437): Grouped `Maybe` tests plus added another `Range` test
- [438](https://github.com/alecthomas/voluptuous/pull/438): Extend tests for `Schema` with empty list or dict

**New**:
- [433](https://github.com/alecthomas/voluptuous/pull/433): Add Python 3.9 support

**Fixes**:
- [431](https://github.com/alecthomas/voluptuous/pull/431): Fixed typos + made spelling more consistent
- [411](https://github.com/alecthomas/voluptuous/pull/411): Ensure `Maybe` propagates error information
- [434](https://github.com/alecthomas/voluptuous/pull/434): Remove value enumeration when validating empty list

**Changes**:
- n/a

**New**:
- [368](https://github.com/alecthomas/voluptuous/pull/368): Allow a discriminant field in validators

**Fixes**:
- [420](https://github.com/alecthomas/voluptuous/pull/420): Fixed issue with 'required' not being set properly and added test
- [414](https://github.com/alecthomas/voluptuous/pull/414): Handle incomparable values in Range
- [427](https://github.com/alecthomas/voluptuous/pull/427): Added additional tests for Range, Clamp and Length + catch TypeError exceptions

Updated the folder structure and added more validators.