fix docs

.readthedocs.yml

@@ -5,17 +5,20 @@
 # Required
 version: 2
 
-# Build documentation in the docs/ directory with Sphinx
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "mambaforge-4.10"
+
+# Build documentation in the doc/ directory with Sphinx
 sphinx:
   configuration: docs/conf.py
   fail_on_warning: false
 
-# Optionally build your docs in additional formats such as PDF and ePub
-formats: []
+# Build the doc in offline formats
+formats:
+  - pdf
+  - htmlzip
 
-python:
-  version: 3.7
-  install:
-    - requirements: dev-requirements.txt
-    - method: pip
-      path: .
+conda:
+  environment: dev-environment.yml