Fix readthedocs builds

The changes in 5648f66 broke the readthedocs build due to "missing
metadata". I believe this should resolve that.

Author: oschwald

.readthedocs.yaml

@@ -7,3 +7,8 @@ build:
 
 sphinx:
   configuration: docs/conf.py
+
+python:
+  install:
+    - method: pip
+      path: .