Pluggable setup process

[ipmb]

Would you consider a PR which made the setup process (currently supporting pip/setuptools) pluggable so projects using alternate build systems could work with shiv? I'm interested in poetry, but others like flit or pipenv could be supported as well.

[ipmb]

On second thought, waiting for pypa/pip#5407 is probably the way to go.

[lorencarvalho]

@ipmb I'm also keeping a keen eye on the pep517 pip PR :)