Changes:

• Added unit tests for all class with a coverage of 99,97%
• Dropped support for PHP < 7.4 and added support for PHP 8
• Type hints and return types everywhere + strict type checking
• The minimum version of doctrine/orm is now 2.3 (the unit tests showed that this is the lowest version this library supports)
• Moved integration tests into a separate test suite
• Remove the Docker stuff from the integration test to simplify those. Docker-Compose is now used to run the integration tests
• Removed TravisCI and use Github actions to run the tests
• Replace Phpstan with Psalm, which actually gets better results and fixed errors found