0.12.1

Changes:

• #435: Extended a few tests (Required and In)
• #425: Improve error message for In and NotIn
• #436: Add sorted() for In and NotIn + fix tests
• #437: Grouped Maybe tests plus added another Range test
• #438: Extend tests for Schema with empty list or dict

New:

• #433: Add Python 3.9 support

Fixes:

• #431: Fixed typos + made spelling more consistent
• #411: Ensure Maybe propagates error information
• #434: Remove value enumeration when validating empty list