ci: Update Docker container to Ubuntu 23.10

This removes Python 2.

.gitlab-ci/Dockerfile

@@ -1,7 +1,7 @@
 # The image is also used for libxslt, that's why we need git and
 # libgcrypt-dev.
 
-FROM ubuntu:22.10
+FROM ubuntu:23.10
 ENV DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && \
     apt-get upgrade -y && \
@@ -10,7 +10,7 @@ RUN apt-get update && \
         autoconf automake libtool pkg-config \
         make gcc clang llvm \
         zlib1g-dev liblzma-dev libgcrypt-dev \
-        python2-dev python3-dev \
+        python3-dev \
         cmake meson
 WORKDIR /tests
 RUN curl https://www.w3.org/XML/Test/xmlts20080827.tar.gz |tar xz