MAINT: Change installation to ``setup.cfg`` for pip > 19

[oesteban]

Comes from #93

[effigies]

Should set a minimum setuptools of 30.3.0 in setup_requires and pyproject.toml. This should get us into a place where having a recent pip OR a recent setuptools will install correctly, and an understandable error will be printed if neither is new enough.

There was a little discussion on nipy/nibabel#764. See the full comparison (nipy/nibabel@935079b...0282bb9) for all modifications.