Bump version to 0.11.3

Bump version number to 0.11.2

Bump version number to 0.11.1

Bump version number to 0.11.0 because of the new PEP 561 compatibility

Replace travis badge with GitHub-Actions badge

Add support for enums, bump version to 0.9.0

Bump version number to 0.8.1

Merge pull request #17 from Dobiasd/taggedconverters

Distinguish between optional and mandatory converters

Add Python 3.8 to docker-compose-based tests

Fix linter complaints