Add black to CI.

Author: wRAR

.github/workflows/build.yml

@@ -27,6 +27,9 @@ jobs:
         - python-version: 3.9
           env:
             TOXENV: security
+        - python-version: 3.9
+          env:
+            TOXENV: black
 
     steps:
     - uses: actions/checkout@v2

tox.ini

@@ -4,7 +4,7 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27, pypy, py35, py36, py37, py38, pypy3, docs
+envlist = py27, pypy, py35, py36, py37, py38, pypy3, docs, security, flake8, pylint, black
 
 [testenv]
 deps =
@@ -37,6 +37,12 @@ deps =
 commands =
     pylint conftest.py docs setup.py tests w3lib
 
+[testenv:black]
+deps =
+    black
+commands =
+    black --check {posargs:conftest.py setup.py tests w3lib}
+
 [docs]
 changedir = docs
 deps = -rdocs/requirements.txt