Python 3.9 support in Singer SDK
We implemented #43 (closed) which made it explicit that Singer SDK did not support Python 3.9, but now that we have more tests and a proper test matrix in CI, we could start working towards Python 3.9 support.
Related to meltano#2545 (closed) on the meltano repo.
As a first step, we can simply add 3.9 to the test matrix and see what errors are generated.