Try updating RtD build

Author: znicholls

.readthedocs.yaml

@@ -9,11 +9,10 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.11"
+    python: "mambaforge-22.9"
   jobs:
     post_install:
-      # RtD seems to be not happy with pdm installs,
-      # hence use pip directly instead.
+      - which python
       - python -m pip install -r requirements-docs-locked.txt
       - python -m pip list
     pre_build:
@@ -22,3 +21,6 @@ build:
 mkdocs:
   configuration: mkdocs.yml
   fail_on_warning: true
+
+conda:
+  environment: environment-docs-conda-base.yml

environment-docs-conda-base.yml

@@ -0,0 +1,10 @@
+name: fortran-compilers
+
+channels:
+  - conda-forge
+  - defaults
+
+dependencies:
+  - python=3.11
+  - gcc
+  - gfortran