Add explicit isort config for build package. (#966)

This is a hack to workaround an obscure edge case. As of tox>=4, running
`tox -e py` generates a `build/` directory in the project root. That
confuses isort into thinking that `build` is a first-party package,
which sorts it in the same section as `twine`. This forces `build` to be
recognized as ta third-party package.

.isort.cfg

@@ -3,4 +3,5 @@ profile=black
 force_single_line=True
 single_line_exclusions=typing
 default_section=THIRDPARTY
+known_third_party=build
 known_first_party=twine,helpers