Add CI job "black" for code format checks

The CI job will fail, if a file needs to be reformatted.
Only files already well formatted will be checked at the moment.

.gitlab-ci.yml

@@ -217,6 +217,36 @@ lint_mypy:
     - mypy
 
 
+black:
+  image: python:slim
+  script:
+    - pip install black
+    - black --check --diff --color --skip-string-normalization
+        examples/fdroid_extract_repo_pubkey.py
+        examples/makebuildserver.config.py
+        fdroid
+        fdroidserver/exception.py
+        fdroidserver/gpgsign.py
+        fdroidserver/lint.py
+        fdroidserver/mirror.py
+        fdroidserver/net.py
+        fdroidserver/readmeta.py
+        fdroidserver/signindex.py
+        fdroidserver/tail.py
+        tests/build.TestCase
+        tests/deploy.TestCase
+        tests/exception.TestCase
+        tests/import.TestCase
+        tests/import_proxy.py
+        tests/init.TestCase
+        tests/install.TestCase
+        tests/key-tricks.py
+        tests/lint.TestCase
+        tests/metadata.TestCase
+        tests/ndk-release-checksums.py
+        tests/rewritemeta.TestCase
+
+
 fedora_latest:
   image: fedora:latest
   only: