chore(deps): bump python from 3.9-bullseye to 3.13-bullseye in /docker

Bumps python from 3.9-bullseye to 3.13-bullseye.

---
updated-dependencies:
- dependency-name: python
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

docker/base.Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.9-bullseye
+FROM python:3.13-bullseye
 LABEL maintainer="IETF Tools Team <tools-discuss@ietf.org>"
 
 ENV DEBIAN_FRONTEND=noninteractive