.gitlab-ci.yml: no need to pin sphinx package anymore
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index a7a4629..ecc0641 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -168,7 +168,7 @@
 docs:
   stage: test
   script:
-  - pip3 install sphinx==1.7.1
+  - pip3 install sphinx
   - pip3 install sphinx-click
   - pip3 install sphinx_rtd_theme
   - cd dist && ./unpack.sh && cd buildstream