CI: use the ci-fairy image and a venv for the black job

The venv is needed now because black now requires a newer 'packaging'
and fails to installed with:
   ERROR: Cannot uninstall 'packaging'. It is a distutils installed
   project and thus we cannot accurately determine which files belong to
   it which would lead to only a partial uninstall.

So let's use a venv and while we're doing this switch to use the
ci-fairy image so we don't need to worry about docker limits.

.gitlab-ci.yml

@@ -129,10 +129,13 @@ check-merge-request:
 # Format anything python with python-black
 #
 python-black:
-  image: golang:alpine
+  extends:
+    - .fdo.ci-fairy
   stage: prep
   before_script:
-    - apk add python3 py-pip black
+    - python3 -m venv venv
+    - source venv/bin/activate
+    - pip3 install black
   script:
     - black --check --diff .
 

.gitlab-ci/ci.template

@@ -139,10 +139,13 @@ check-merge-request:
 # Format anything python with python-black
 #
 python-black:
-  image: golang:alpine
+  extends:
+    - .fdo.ci-fairy
   stage: prep
   before_script:
-    - apk add python3 py-pip black
+    - python3 -m venv venv
+    - source venv/bin/activate
+    - pip3 install black
   script:
     - black --check --diff .