docs: Use mambaforge python tools on rtd

The normal python build tools apparently don't ship conda or mamba,
and we have to use one of the special ones.

See also https://docs.readthedocs.io/en/stable/guides/conda.html

[skip ci]

Author: st-bender

.readthedocs.yml

@@ -3,7 +3,7 @@ version: 2
 build:
     os: "ubuntu-20.04"
     tools:
-        python: "3"
+        python: "mambaforge-4.10"
 
 conda:
     environment: .rtd-environment.yml