Initial commit of Flake8 as a development requirement

CONTRIBUTING.md

@@ -19,3 +19,11 @@ python -m piptools compile --output-file=requirements.txt
 ```sh
 python -m pip install -e ".[dev]"
 ```
+
+### How to run a linter?
+
+Run [Flake8](https://flake8.pycqa.org/):
+
+```sh
+python -m flake8
+```

setup.cfg

@@ -0,0 +1,12 @@
+[flake8]
+# Recommend matching the black line length (default 88),
+# rather than using the flake8 default of 79:
+max-line-length = 88
+
+extend-ignore =
+    # Currently, `pycodestyle` gives false positives on the spaces black uses for slices,
+    # which flake8 reports as “Whitespace before ':' (E203)”. This is why we chose to ignore
+    # E203 until a version of flake8 is released with a fix of:
+    # https://github.com/PyCQA/pycodestyle/issues/373 is closed.
+    # https://www.flake8rules.com/rules/E203.html
+    E203,

setup.py

@@ -5,6 +5,7 @@ requirements = [
 ]
 
 dev_requirements = [
+    "flake8",
     "pip-tools",
 ]