Add a pre-commit hook for black

Done as [documented][0]. Since one has to have `black` installed in
order to make use of this hook, I added `black` to the dev dependencies.
The "pyproject.toml" file is necessary, because that's the only place
that `black` will look for configuration parameters. Last but not least,
the `[build-system]` section in "pyproject.toml" is because we still use
"setup.py" for packaging and `pip` would get confused by the presence of
"pyproject.toml". The `[build-system]` section just tells it to look for
"setup.py".

[0]: https://black.readthedocs.io/en/stable/version_control_integration.html#version-control-integration

.pre-commit-config.yaml

@@ -18,3 +18,8 @@ repos:
     rev: master
     hooks:
       - id: flake8
+  - repo: https://github.com/psf/black
+    rev: stable
+    hooks:
+      - id: black
+        language_version: python3 # Should be a command that runs python3.6+

pyproject.toml

@@ -0,0 +1,17 @@
+[tool.black]
+line-length = 79
+target-version = ["py36", "py37", "py38"]
+include = '\.pyi?$'
+exclude = '''
+/(
+    \.git
+  | \.tox
+  | build
+  | ci
+  | dist
+)/
+'''
+
+[build-system]
+requires = ["setuptools>=41.0", "wheel"]
+build-backend = "setuptools.build_meta"

setup.py

@@ -87,7 +87,7 @@ def read(*names, **kwargs):
         # eg: 'aspectlib==1.1.1', 'six>=1.7',
     ],
     extras_require={
-        "dev": ["pytest", "tox"]
+        "dev": ["black", "pytest", "tox"]
         # eg:
         #   'rst': ['docutils>=0.11'],
         #   ':python_version=="2.6"': ['argparse'],