add black to pre-commit

.pre-commit-config.yaml

@@ -0,0 +1,6 @@
+repos:
+  - repo: https://github.com/psf/black
+    rev: 20.8b1 # Replace by any tag/version: https://github.com/psf/black/tags
+    hooks:
+      - id: black
+        language_version: python3 # Should be a command that runs python3.6+

README.md

@@ -123,6 +123,7 @@ cd jupyterlab-git
 
 # Install the server extension in development mode and enable it
 pip install -e .[test]
+pre-commit install
 jupyter serverextension enable --py jupyterlab_git --sys-prefix
 
 # Build and install your development version of the extension

setup.py

@@ -88,6 +88,7 @@ def runPackLabextension():
             "pytest-asyncio",
             "jupyterlab~=2.0",
             "black",
+            "pre-commit",
         ],
     },
 )