Add basic pre-commit configuration for tuf/api/*

Add optional pre-commit configuration to install and run
auto-formatters when committing new code to tuf/api/*.
Auto-formatters include:
- trailing-whitespace
- end-of-file-fixer
- black
- isort

This commit also adds pre-commit to the dev dependencies
and updates the contributor instructions accordingly.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

.pre-commit-config.yaml

@@ -0,0 +1,23 @@
+# See https://pre-commit.com for more information
+# See https://pre-commit.com/hooks.html for more hooks
+files: ^tuf/api/
+repos:
+  - repo: https://github.com/pre-commit/pre-commit-hooks
+    rev: v3.4.0
+    hooks:
+      - id: trailing-whitespace
+      - id: end-of-file-fixer
+
+  - repo: https://github.com/psf/black
+    rev: 20.8b1
+    hooks:
+      - id: black
+        # TODO: configure args in pyproject.toml (see #1161)
+        args: [--line-length=80, tuf/api]
+
+  - repo: https://github.com/pycqa/isort
+    rev: 5.7.0
+    hooks:
+      - id: isort
+         # TODO: configure args in pyproject.toml (see #1161)
+        args: [--line-length=80, --profile=black, tuf/api]

docs/CONTRIBUTORS.rst

@@ -105,6 +105,16 @@ To work on the TUF project, it's best to perform a development install.
     $ pip install -r requirements-dev.txt
 
 
+Auto-formatting
+===============
+
+New TUF code should be auto-formatted with `black <https://black.readthedocs.io/>`_
+and `isort <https://pycqa.github.io/isort>`_. The tasks can be installed as
+`pre-commit <https://pre-commit.com/>`_ git hooks with the following command.
+::
+    $ pre-commit install
+
+
 Testing
 =======
 

requirements-dev.txt

@@ -1,5 +1,6 @@
 # Install tuf in editable mode and requirements for local testing with tox,
 # and also for running test suite or individual tests manually
 tox
+pre-commit
 -r requirements-test.txt
 -e .