Fix offline builds

pip 19 added support for something called `pyproject.toml`. Several packages have been setting the option `build-backend = "setuptools.build_meta”` (bcrypt, attrs, jaraco) which seems to be the root of the problem when building from source. Until the community sorts this out I’m inclined to avoid pip 19.

requirements/requirements_setup_requires.txt

@@ -5,3 +5,5 @@ pytest-runner
 isort
 virtualenv
 m2r
+flit
+intreehooks