use python limited api

[electronstudio]

Reasons to use it:

1. We would need to build far fewer wheels.

Reasons we didnt use it before:

1. CFFI says it might degrade performance
2. We tried it and sometimes we had reports of wheels that didn't load.

But we now have automated testing to verify wheels at least load so that should not be a problem?

[electronstudio]

I saw a comment this week that the 'stable' ABI still isn't really stable, so maybe not worth the risk.