Remove use of stevedore

[ssbarnea]

stevedore lacks types and is not actively maintained, we need to get rid of it.

Any PR is welcomed. My guess is that pluggy might be a good guess.

I am not sure how many are using extensions, as if nothing happens we might just remove the support for extensions and bump the major version, at least until someone using extensions readds the support.

[johnsom]

stevedore is a widely used part of OpenStack and maintained. It has regular releases, so I'm not sure I agree with the "not actively maintained" statement.