BLD: explicitly specify build-backend

[neutrinoceros]

I'm setting the implicit value build-backend explicitly, as recommended.
setuptools' doc also clarifies that wheel requirement isn't needed as it's implied by using the setuptools_meta backend, so I'm removing it. See https://setuptools.pypa.io/en/latest/userguide/quickstart.html#basic-use

You should only include wheel in requires if you need to explicitly access it during build time (e.g. if your project needs a setup.py script that imports wheel).