Fix bug where worker terminates when max_messages is not set

Fix bug in enqueue unit test

Update CI/CD and use dataclasses instead of namedtuples

Bump up dependency versions and flush producer on Queue.enqueue

Revamp the library for 2.0.0

Add contributing page to the documentation

Add license and tests_require in setup.py

Update docstring for Worker class

Update documentation

Add enqueue_with_key method to Queue class