gitlab-ci: Keep Readthedocs theme version in variable

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

.gitlab-ci.yml

@@ -5,6 +5,7 @@ variables:
   GITLAB_CI_IMAGE_PYTHON: 'python:3'
   GITLAB_CI_PYPI_DOCKER_COMPOSE: 'docker-compose~=1.23.0'
   GITLAB_CI_PYPI_TOX: 'tox~=3.8.0'
+  GITLAB_CI_PYPI_SPHINX_RTD_THEME: 'sphinx_rtd_theme~=0.4.0'
 stages:
   - static
   - build
@@ -33,7 +34,7 @@ docs:
   stage: build
   image: ${GITLAB_CI_IMAGE_PYTHON}
   before_script:
-    - pip install sphinx_rtd_theme
+    - pip install "$GITLAB_CI_PYPI_SPHINX_RTD_THEME"
   script:
     - cd docs
     - make html SPHINXOPTS="-W"