ci: changing docs python version to 3.8

Author: Lachlan Grose

.github/workflows/documentation.yml

@@ -14,7 +14,7 @@ jobs:
           auto-activate-base: true
           installer-url: https://github.com/conda-forge/miniforge/releases/download/4.10.1-0/Miniforge-pypy3-4.10.1-0-Linux-x86_64.sh
           channels: anaconda,conda-forge,loop3d,conda-forge/label/cf202003
-          python-version: 3.9
+          python-version: 3.8
       - name: Build documentation 
         shell: bash -l {0}
         env: