Add a pre-commit config file

and document it briefly in the README file.

This comes with a few checkers:
- flake8
- trailing-whitespace
- check-json
- check-yaml
- codespell

Also include commented checkers:
- black: disabled since we are not ready for this yet,
- pydocstyle: disabled since we have way too many pydoctyle errors for now...

See https://pre-commit.com/ for more details.

Related to T1881.

.pre-commit-config.yaml

+repos:
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v1.2.3
+  hooks:
+  - id: trailing-whitespace
+  - id: flake8
+  - id: check-json
+  - id: check-yaml
+
+- repo: https://github.com/codespell-project/codespell
+  rev: v1.15.0
+  hooks:
+  - id: codespell
+
+# unfortunately, we are far from being able to enable this...
+# - repo: https://github.com/PyCQA/pydocstyle.git
+#   rev: 4.0.0
+#   hooks:
+#   - id: pydocstyle
+#     name: pydocstyle
+#     description: pydocstyle is a static analysis tool for checking compliance with Python docstring conventions.
+#     entry: pydocstyle --convention=google
+#     language: python
+#     types: [python]
+
+# black requires py3.6+
+#- repo: https://github.com/python/black
+#  rev: 19.3b0
+#  hooks:
+#  - id: black
+#    language_version: python3
+#- repo: https://github.com/asottile/blacken-docs
+#  rev: v1.0.0-1
+#  hooks:
+#  - id: blacken-docs
+#    additional_dependencies: [black==19.3b0]

README.md

@@ -23,7 +23,7 @@ To set up your development environment:
 
 This will install every Python package needed to run this package's tests.
 
-Unit tests can be executed using [pytest](https://docs.pytest.org).
+Unit tests can be executed using [pytest][2] or [tox][3].
 
 ```
 (swh) user@host:~/swh-environment/swh-core$ pytest
@@ -46,8 +46,21 @@ swh/core/tests/test_utils.py .......                                      [100%]
 ===================== 89 passed, 9 warnings in 6.94 seconds =====================
 ```
 
+Note: this git repository uses [pre-commit][4] hooks to ensure better and more
+consistent code. It should already be installed in your virtualenv (if not,
+just type `pip install pre-commit`). Make sure to activate it in your local
+copy of the git repository:
+
+```
+(swh) user@host:~/swh-environment/swh-core$ pre-commit install
+pre-commit installed at .git/hooks/pre-commit
+```
+
 Please read the [developer setup manual][5] for more information on how to hack
 on Software Heritage.
 
-[2]: https://docs.softwareheritage.org/devel/developer-setup.html
 [1]: https://virtualenv.pypa.io
+[2]: https://docs.pytest.org
+[3]: https://tox.readthedocs.io
+[4]: https://pre-commit.com
+[5]: https://docs.softwareheritage.org/devel/developer-setup.html

requirements-test.txt

@@ -3,3 +3,4 @@ pytest < 4
 pytest-postgresql
 requests-mock
 hypothesis >= 3.11.0
+pre-commit