use Debian stable when specifying Kwant debian upstream

docker/Dockerfile.debian

@@ -6,7 +6,7 @@ ENV LC_ALL C.UTF-8
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gnupg dirmngr apt-transport-https ca-certificates curl software-properties-common
 
-RUN echo "deb http://downloads.kwant-project.org/debian/ stretch-backports main" >> /etc/apt/sources.list && \
+RUN echo "deb http://downloads.kwant-project.org/debian/ stable main" >> /etc/apt/sources.list && \
     apt-key adv --no-tty --keyserver pool.sks-keyservers.net --recv-key C3F147F5980F3535 && \
     apt-get update && apt-get install -y --no-install-recommends \
         # all the hard non-Python dependencies