[CI] add black hook

.gitlab-ci.yml

 stages:
+  - style
   - build
   - test_library
   - test_notbooks
@@ -10,6 +11,19 @@ variables:
   no_proxy: .esrf.fr,localhost
   ORANGE_WEB_LOG: 'False'
 
+
+# style
+check_style:
+  stage: style
+  image: docker-registry.esrf.fr/dau/tomwer:python3.8_buster_pyqt5_sphinx
+  before_script:
+    - pip install black
+  script:
+    - pip install -r requirements.txt
+    # run black
+    - LC_ALL=C.UTF-8 black --check --safe .
+
+
 # build
 doc:
   stage: build

.pre-commit-config.yaml

+repos:
+-   repo: https://github.com/psf/black
+    rev: 18.6b4
+    hooks:
+    - id: black
+      language_version: python3

pre-commit.sh

+#!/bin/bash
+# This script installs a pre-commit hook on the local repository
+# It allows black to run before any git commit.
+
+# Install pre-commit, with a local install fallback
+pip install --quiet --upgrade pre-commit 2>/dev/null \
+    || pip install --quiet --user --upgrade pre-commit
+
+# Install the pre-commit hook on the local repository
+pre-commit install