Add CI matrix tests against all supported Python versions #51
Description
Context:
I've done that in the JS and Ruby SDKs - in fact, I define the testing matrix as
- all supported versions of the PDP with the latest version of the language; plus
- the latest version of the PDP with all supported versions of the language; plus
- the current prerelease of the PDP with the latest version of the language (but failure of this one doesn't break the build).