gitlab-ci: keep pip cache between jobs

.gitlab-ci.yml

@@ -10,8 +10,8 @@ test:
   - pushd /usr/lib/python3/dist-packages/xapian && ln -s _xapian* _xapian.so && popd
   - export PYTHONPATH="/usr/lib/python3/dist-packages:$PYTHONPATH"
   - python -c 'import xapian' # Fail immediately if there is a problem with xapian
-  - pip install -r requirements.txt
-  - pip install coverage
+  - pip install -U -r requirements.txt
+  - pip install -U coverage
   - mkdir -p /dev/shm/search_indexes
   - ln -s /dev/shm/search_indexes sith/search_indexes
   - ./manage.py compilemessages
@@ -20,6 +20,11 @@ test:
   - coverage report
   - cd doc
   - make html # Make documentation
+  variables:
+    PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip_tests"
+  cache:
+    paths:
+      - .cache/pip_tests
   artifacts:
     paths:
       - coverage_report/
@@ -27,5 +32,10 @@ test:
 black:
   stage: test
   script:
-    - pip install black
+    - pip install -U black
     - black --check .
+  variables:
+    PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip_black"
+  cache:
+    paths:
+      - .cache/pip_black