SimpleEnvelope._DEFAULT_PAYLOAD_TYPE should be public?

[jku]

ngclient accesses SimpleEnvelope._DEFAULT_PAYLOAD_TYPE.

flake8-self linter complains about this, I think correctly. If we use the value from a different module, it should be public

CC @lukpueh for comment

[lukpueh]

I suppose I didn't see a reason to expose it to every user of the API. But flake8 is right, that ngclient is also just a user of that API. I'm with with making the variable public.

[h4l0gen]

Hi, I would like to work on it. I will create PR. Thank you 👍