lock black to known good

Author: jschlyter

setup.py

@@ -58,7 +58,7 @@
     extras_require={
         "testing": tests_requires,
         "docs": ["Sphinx", "sphinx-autobuild", "alabaster"],
-        "quality": ["isort>=5.0.2", "black"],
+        "quality": ["isort>=5.0.2", "black==19.10b0"],
     },
     scripts=glob.glob("script/*.py"),
     entry_points={