Use contextvars for managing dependency injection

[andrewwhitehead]

If we increase the minimum required Python version to 3.7, then we can support the use of contextvars. I'm not sure what an implementation would look like yet, but this should allow us to easily inject dependencies without explicitly passing in Context-type objects every time.

[esune]

I believe the minimum version supported by ACA-Py is currently 3.9: what work would be required to complete this issue, a refactor of the current code?

[WadeBarnes]

Correct min supported python version for ACA-Py is 3.9