[ci] #335: Set Py 3.8 as default image

Since tools are installed on

.gitlab-ci.yml

@@ -6,7 +6,7 @@ stages:
 
 variables:
   DOCKER_IMAGE: "registry.duniter.org/docker/python3/poetry"
-  PYTHON_VERSION: "3.7"
+  PYTHON_VERSION: "3.8"
 
 image: $DOCKER_IMAGE/$PYTHON_VERSION:latest
 
@@ -34,7 +34,6 @@ build:
 format:
   extends: .code_changes
   stage: checks
-  image: $DOCKER_IMAGE/3.8:latest
   script:
     - black --check silkaj tests
 
@@ -55,6 +54,8 @@ tests-3.6:
 tests-3.7-coverage:
   extends: .tests
   tags: [mille]
+  variables:
+    PYTHON_VERSION: "3.7"
   script:
     - poetry install
     - poetry run pytest --cov silkaj --cov-report html:cov_html
@@ -67,8 +68,6 @@ tests-3.7-coverage:
 tests-3.8:
   extends: .tests
   tags: [redshift]
-  variables:
-    PYTHON_VERSION: "3.8"
 
 tests-3.9:
   extends: .tests