Add pyproject.toml to avoid re-running black on ropetest

black currently has poor multi-line string treatment for dedent()-ed code.

I've ran aneeshusa's black branch psf/black#1879
on ropetest instead, which leaves dedent()-ed lines alone; however
most people likely will be running mainline black which would have
mucked these formatting , so we're adding an exclusion rule in
pyproject.toml prevent people from auto-formatting ropetest.

pyproject.toml

@@ -0,0 +1,4 @@
+[tool.black]
+
+target-version = ['py27', 'py33', 'py34', 'py35', 'py36', 'py37', 'py38', 'py39']
+exclude = 'ropetest'