build: Remove wheel and attrs from build-system requires (#1788)

* Remove wheel from build-system requires as it is declared by
PEP 517's get_requires_for_build_sdist and so already covered.
   - c.f. https://www.python.org/dev/peps/pep-0517/
* Remove attrs as not required at build time.

Author: matthewfeickert

pyproject.toml

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