Use extra requirements to build readthedocs

Fixes: containers#532

Signed-off-by: Nicola Sella <[email protected]>

Author: inknos

.readthedocs.yaml

@@ -21,7 +21,10 @@ build:
 # https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
   install:
-  - requirements: requirements.txt
+    method: pip
+    path: .
+    extra_requirements:
+      - docs
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: