Reinstate tox -e lint

Reintroduce the lint environment after 5116305
to just run the cheap linting tools to make it easier for developers to
have tools at hand.

CONTRIBUTING.md

@@ -67,8 +67,8 @@ You can run `tox` with the following arguments:
 `black` and `isort` are executed when `tox -e lint` is run. The reported errors can be tedious to fix manually.
 An easier way to do so is:
 
-1. Run `.tox/lint-some-package/bin/black .`
-2. Run `.tox/lint-some-package/bin/isort .`
+1. Run `.tox/lint/bin/black .`
+2. Run `.tox/lint/bin/isort .`
 
 Or you can call formatting and linting in one command by [pre-commit](https://pre-commit.com/):
 

tox.ini

@@ -1189,6 +1189,17 @@ changedir = docs
 commands =
   sphinx-build -E -a -W -b html -T . _build/html
 
+[testenv:lint]
+basepython: python3
+recreate = True
+deps =
+  -r dev-requirements.txt
+
+commands =
+  black --config {toxinidir}/pyproject.toml {{toxinidir}} --diff --check
+  isort --settings-path {toxinidir}/.isort.cfg {{toxinidir}} --diff --check-only
+  flake8 --config {toxinidir}/.flake8 {toxinidir}
+
 [testenv:spellcheck]
 basepython: python3
 recreate = True