tests: add python3-venv to debian10.docker

This is needed to be able to add a venv-building step to 'make check';
the clang-user job in particular needs this to be able to run
check-unit.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220526000921.1581503-9-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/docker/dockerfiles/debian10.docker

@@ -34,4 +34,5 @@ RUN apt update && \
         python3 \
         python3-sphinx \
         python3-sphinx-rtd-theme \
+        python3-venv \
         $(apt-get -s build-dep --arch-only qemu | egrep ^Inst | fgrep '[all]' | cut -d\  -f2)