build: add pre-commit configuration to run black (#49784)

.pre-commit-config.yaml

@@ -0,0 +1,8 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+repos:
+-   repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+    - id: black
+      args: ['--target-version', 'py35', '--skip-string-normalization', '--line-length', '110']

README

@@ -32,7 +32,12 @@ in the CHRONO_SETTINGS_FILE environment variable.
 Code Style
 ----------
 
-black --target-version py35 --skip-string-normalization --line-length 110
+black is used to format the code, using thoses parameters:
+
+    black --target-version py35 --skip-string-normalization --line-length 110
+
+There is .pre-commit-config.yaml to use pre-commit to automatically run black
+before commits. (execute `pre-commit install` to install the git hook.)
 
 
 Tests