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 while doing
all its other cleanups.

However most people likely will be running mainline black which would
have mucked the formatting in these files, so I've also added an
exclusion rule in pyproject.toml to prevent people from accidentally
auto-formatting ropetest again.

Until aneeshusa's branch are merged into mainline black, or black has a
proper solution for dedent()-ed code, be careful of running black on
ropetest.

pyproject.toml

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