update readthedocs.yml to mamba (#393)

use mambaforge to force readthedocs to use conda environments  issue from readthedocs/readthedocs.org#8595

Author: Haaroon

.readthedocs.yaml

@@ -9,7 +9,7 @@ version: 2
 build:
   os: ubuntu-20.04
   tools:
-    python: "3.9"
+    python: "mambaforge-4.10"
 
 conda:
   environment: .environment.yml