Update readthedocs config

Read the Docs configuration was introduced to require a higher then
default version of Python. This worked then, but apparently now we
also need to explicitly state that our package needs to be installed
in order to build its docs. Fixes datalad#117

.readthedocs.yaml

@@ -7,6 +7,14 @@ version: 2
 
 # Set the version of Python and other tools you might need
 build:
-  os: ubuntu-20.04
+  os: "ubuntu-22.04"
   tools:
     python: "3.8"
+
+sphinx:
+  configuration: docs/source/conf.py
+
+python:
+  install:
+    - method: pip
+      path: .