build: Require setuptools v42.0.0+ for stability (#1783)

* setuptools releases prior to v42.0.0 had stability issues with
pyproject.toml or didn't support PEP 517 and so couldn't support
pyproject.toml.
   - c.f. https://www.python.org/dev/peps/pep-0517/

Co-authored-by: Henry Schreiner <[email protected]>

Author: matthewfeickert

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 # Minimum requirements for the build system to execute.
-requires = ["wheel", "setuptools>=30.3.0", "attrs>=17.1", "setuptools_scm[toml]>=3.4"]
+requires = ["wheel", "setuptools>=42.0.0", "attrs>=17.1", "setuptools_scm[toml]>=3.4"]
 build-backend = "setuptools.build_meta"
 
 [tool.setuptools_scm]