.gitlab-ci: install doc/sphinx/requirements.txt

Install all requirements according to doc/sphinx/requirements.txt in the
virtual environment used for testing 'make htmldocs'.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

.azure-pipelines.yml

@@ -67,7 +67,11 @@ jobs:
       image: $(ci_runner_image)
       options: $(container_option)
     steps:
-      - script: make htmldocs
+      - script: |
+          virtualenv -p /usr/bin/python3 /tmp/venvhtml
+          . /tmp/venvhtml/bin/activate
+          pip install -r doc/sphinx/requirements.txt
+          make htmldocs
 
   - job: todo
     displayName: 'Search for TODO within source tree'

.gitlab-ci.yml

@@ -122,6 +122,9 @@ htmldocs:
   tags: [ 'all' ]
   stage: testsuites
   script:
+    - virtualenv -p /usr/bin/python3 /tmp/venvhtml
+    - . /tmp/venvhtml/bin/activate
+    - pip install -r doc/sphinx/requirements.txt
     - make htmldocs
 
 # some statistics about the code base