Consider changing the packaging tool

[Cito]

Instead of pipenv, we may want to use a different packaging tool like flit or poetry, and replace setup.py and pipfile with a pyproject.toml file. The main reason is that while pipenv is a nice tool, it is actually intended to package apps, not libraries like graphql-core-next.

Would like to hear thougts and recommendations of others regarding this issue.

[Cito]

I have now switched to poetry. For the time being I still keep the setup.py file though, and will create the source distribution from it, because with poetry I don't see how I can put things (like tests and docs) into the source distribution, but not into the wheel. With distutils, this is possible using the manifest template.