Update .readthedocs.yaml

- fix #280
- use mambaforge installation

.readthedocs.yaml

@@ -5,6 +5,11 @@
 # Required
 version: 2
 
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "mambaforge-22.9"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
    configuration: doc/sphinx/source/conf.py
@@ -13,8 +18,12 @@ sphinx:
 formats:
    - pdf
 
-# Optionally set the version of Python and requirements required to build your docs
+# install package
 python:
-   version: 3.8
    install:
-   - requirements: doc/requirements.txt
+      - method: pip
+        path: .
+
+conda:
+    environment: devtools/conda-envs/test_env.yaml
+