CI: Use python 3.9

Author: sebp

.github/actions/sphinx-build-all/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:slim
+FROM python:3.9-slim
 
 RUN apt-get update \
  && apt-get install -y \
@@ -14,4 +14,4 @@ RUN python3 -m pip install tox
 
 COPY entrypoint.sh /entrypoint.sh
 
-ENTRYPOINT ["/entrypoint.sh"]
+ENTRYPOINT ["/entrypoint.sh"]