Merge pull request #415 from dhellmann/fix-rtd-install

fix installation instructions for rtd

Author: mergify[bot]

.readthedocs.yaml

@@ -11,9 +11,10 @@ sphinx:
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
   install:
-    - requirements: .[docs]
     - method: pip
       path: .
+      extra_requirements:
+        - docs
 
 submodules:
   include: all