fix pipeline needing gcc for dulwich

.gitlab-ci.yml

@@ -10,10 +10,7 @@ cache:
 
 .install_sphinx:
   before_script:
-  - pip install -U sphinx
-  - pip install -U sphinx-theme-pd
-  - pip install -U reno[sphinx]
-
+  - pip install -U -r docs/requirements.txt
 
 test-docs:
   stage: test

docs/requirements.txt

@@ -0,0 +1,5 @@
+reno[sphinx]==3.5.0
+dulwich --global-option='--pure'
+Sphinx==5.1.1
+sphinx-theme-pd==0.8.3
+