enforce black in CI

We do it in a rather convoluted way (ie. tox) but it will allow us to
hook more goodies later, e.g. mypy, unit tests, etc.

.gitlab-ci.yml

+variables:
+  LC_ALL: "C.UTF-8"
+  LANG: "C.UTF-8"
+
+# We use a job template to avoid mindless repetition.  All of our jobs
+# run 'tox', and we vary which tox environment they run or which
+# Python version they use by overriding the 'TOXENV' environment
+# variable or the 'image' job parameter.
+#
+# The name of the template starts with a dot to prevent GitLab from
+# running it as an actual job.
+.tox:
+  image: python:latest
+  before_script:
+    - pip install tox
+  script:
+    - tox
+  variables:
+    TOXENV: py
+    # The environment 'py' uses the version of Python used to invoke tox,
+    # which allows us to test various Python versions by changing the
+    # Docker image version in use.
+
+flake8:
+  extends: .tox
+  variables:
+    TOXENV: flake8
+
+black:
+  extends: .tox
+  variables:
+    TOXENV: black

requirements.txt

+# just delegate install to setup.py for now
+#
+# this exists mostly so we don't have to duplicate test dependencies
+# between tox and setup.py
+-e .

tox.ini

+[tox]
+envlist = flake8,black
+
+[testenv:flake8]
+skip_install = true
+deps = flake8
+commands = flake8 dangerzone-webdav-processor
+
+[testenv:black]
+skip_install = true
+deps = black
+commands = black --diff --check dangerzone-webdav-processor