Add a pre-commit config file

.pre-commit-config.yaml

+repos:
+- repo: https://github.com/pre-commit/pre-commit-hooks
+  rev: v2.4.0
+  hooks:
+  - id: trailing-whitespace
+  - id: flake8
+  - id: check-json
+  - id: check-yaml
+
+- repo: https://github.com/codespell-project/codespell
+  rev: v1.16.0
+  hooks:
+  - id: codespell
+
+- repo: local
+  hooks:
+  - id: mypy
+    name: mypy
+    entry: env DJANGO_SETTINGS_MODULE=swh.deposit.settings.testing mypy
+    args: [swh]
+    pass_filenames: false
+    language: system
+    types: [python]
+
+# 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]