[enh] #132: Introduce Black pre-commit

Introduce .pre-commit-config.yml
Document its usage in CONTRIBUTING.md
Add pre-commit dependency as a dev dependency

.pre-commit-config.yaml

+repos:
+-   repo: https://github.com/psf/black
+    rev: 21.5b0
+    hooks:
+    - id: black

CONTRIBUTING.md

 # Contribute guide
 
-## Release workflow
+## Pre-commit
+We are using [`pre-commit`](https://pre-commit.com/) tool to perform checks on staged changes before committing. Such as black formatting.
+Depending whether you prefer to work ouside or inside Poetry virtual environment, you should stick to one usage or use both to still have this usefull checks.
+`pre-commit` can be used from the installation on your system or from inside Poetry virtual environment where it is installed.
+
+To install the `git-hooks`, run:
+```bash
+pip3 install --user pre-commit
+duniterpy> pre-commit install
+```
 
+```bash
+duniterpy> poetry shell
+(duniterpy-UyTOfZjU-py3.9) pre-commit install
+```
+
+### Black formatting
+We are using [Black](https://github.com/psf/black) formatter tool.
+Run Black on a Python file to format it:
+```bash
+poetry run black duniterpy/file.py
+```
+With `pre-commit`, Black is called on staged files, so the commit should fail in case black would make changes.
+You will have to add Black changes in order to commit your changes.
+
+## Release workflow
 To handle a release, you have to respect this workflow:
 
 * Verify all features and bug fixes are merged in the `dev` branch.

README.md

@@ -73,8 +73,6 @@ poetry run make docs
 * When writing docstrings, use the reStructuredText format recommended by https://www.python.org/dev/peps/pep-0287/#docstring-significant-features
 * Use `make` commands to check the code and the format.
 
-Black, the formatting tool, requires Python 3.6 or higher.
-
 * Install runtime dependencies
 ```bash
 poetry install --no-dev

pyproject.toml

@@ -38,5 +38,6 @@ websocket-client = "^0.57"
 black = {version = "^21.4b2", allow-prereleases = true}
 mypy = "^0.790"
 pylint = "^2.7.2"
+pre-commit = "^2.12.1"
 sphinx = "^3.2.1"
 sphinx_rtd_theme = "^0.5.0"