Remove Bitbucket pipeline step for no-longer-supported Python versions

[nathanjmcdougall]

Motivation
If a python-requires no longer includes a version of python, we can't test it on the pipeline so we should probably remove it. This ties into #42 though, so I am not sure of the exact mechanics.

Summary of desired enhancement
Provide a way for no-longer-supported python versions to stop being tested in Bitbucket CI.

[nathanjmcdougall]

I've put this as a bug for now because it's the possibility of something broken - is this a guarantee that usethis is actually providing though, that config it adds will never break provided you are only using usethis to modify config? If so it needs to be documented. More thought needed on this one.