Enforce Python codestyle with black

Black is an opinionated tool for auto-formatting Python code so we no
longer have to worry about the codestyle.

For the codestyle decisions and discussion, refer to the upstream
documentation [1].

[1] https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html

(cherry picked from commit 5d2b7cab08)

.gitlab-ci.yml

@@ -427,6 +427,19 @@ misc:
     expire_in: "1 day"
     when: on_failure
 
+black:
+  <<: *precheck_job
+  needs: []
+  script:
+    - black $(git ls-files '*.py')
+    - git diff > black.patch
+    - if test "$(git status --porcelain | grep -Ev '\?\?' | wc -l)" -gt "0"; then git status --short; exit 1; fi
+  artifacts:
+    paths:
+      - black.patch
+    expire_in: "1 week"
+    when: on_failure
+
 clang-format:
   <<: *precheck_job
   needs: []

doc/dev/style.md

@@ -855,9 +855,9 @@ name server. However, BIND 9 may use it for its system test
 environment, and in some cases for generating source or documentation
 files which are then committed to to the git repository.
 
-For Python coding, we abide by the Python style guidelines described
-in [PEP8](http://www.python.org/dev/peps/pep-0008/), with a few
-modifications:
+For Python coding, we enforce a common codestyle using the tool
+[black](https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html)
+There are also a few other requirements:
 
 * The `__init__()` method should always be the first one declared in a
   class definition, like so: