CI: use the ansible-mitogen package

ci/Dockerfile

@@ -2,8 +2,7 @@ FROM debian:bullseye
 COPY with-ssh-key /usr/bin/with-ssh-key
 RUN apt-get -q update \
     && env DEBIAN_FRONTEND=noninteractive apt-get install -q -y --no-install-recommends \
-        ansible python3-pip \
-    && pip3 install mitogen \
+        ansible ansible-mitogen \
     && apt-get clean \
     && rm -fr /var/lib/apt/lists/*