Use correct syntax for installing .[docs]

Author: benjaoming

.readthedocs.yml

@@ -15,6 +15,9 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-   version: 3.7
+   version: 3
    install:
-   - requirements: .[docs]
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs