use pep-517 to get brotlicffi to build on macos

Author: graingert

pyproject.toml

@@ -79,6 +79,7 @@ legacy_tox_ini = """
  wheel_build_env = .pkg
  extras = test
  commands = pytest -v -s {posargs}
+ install_command = python -I -m pip install --use-pep517 {opts} {packages}
 
  [testenv:release]
  deps =