style: integrate black into tox.ini and gitlab-ci

.gitlab-ci.yml

@@ -19,6 +19,10 @@ check-manifest:
   stage: tests
   script: tox -e check-manifest
 
+black:
+  stage: tests
+  script: tox -e black
+
 trigger-other-pipelines:
   only:
     refs:

tox.ini

 [tox]
-envlist=py3,check-manifest
+envlist=py3,check-manifest,black
 
 [testenv]
 deps =
@@ -26,3 +26,17 @@ deps =
   check-manifest
 commands=
   {envpython} -m check_manifest {toxinidir}
+
+[testenv:black]
+basepython = python3
+skip_install = true
+deps =
+  black >= 19.10b0
+commands = black --check .
+
+[testenv:black-run]
+basepython = python3
+skip_install = true
+deps =
+  black >= 19.10b0
+commands = black .