fix nex readthedocs requirements (#414)

 Update to new readthedocs requirements 

---------

Co-authored-by: Jacques Dainat <jacques.dainat@ird.fr>

.readthedocs.yml

@@ -5,6 +5,12 @@
 # Required
 version: 2
 
+# Set the OS, Python version and other tools you might need
+build:
+   os: ubuntu-22.04
+   tools: 
+      python: "3.7"
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
    configuration: docs/conf.py
@@ -13,8 +19,9 @@ sphinx:
 #formats:
 #   - pdf
 
-# Optionally set the version of Python and requirements required to build your docs
+# Optional but recommended, declare the Python requirements required
+# to build your documentation
+# See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
-   version: 3.7
    install:
-   - requirements: docs/requirements.txt
+      - requirements: docs/requirements.txt