Update build_docs.yml (#109)

Author: C-Achard

.github/workflows/build_docs.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-        python-version: [3.8]
+        python-version: [3.9]
     steps:
     - uses: actions/checkout@v2