move back pin on ruamel.yaml, seems to be causing doc issues

Author: walshmm

.readthedocs.yaml

@@ -21,7 +21,7 @@ build:
       # Install the package in editable mode using pixi
       - $HOME/.pixi/bin/pixi run --environment docs pip install -e .
       # Install docs dependencies in the RTD Python environment too
-      - pip install erdantic versioningit sphinx_rtd_theme sphinxcontrib-mermaid types-pyyaml h5py numpy matplotlib ruamel.yaml
+      - pip install erdantic versioningit sphinx_rtd_theme sphinxcontrib-mermaid types-pyyaml h5py numpy matplotlib ruamel.yaml=0.18.16
 
 sphinx:
   builder: html