Debian: we now need libpq-dev.

psycopg 2.8 is now out, which means that the C library gets built from source,
so we now need libpq-dev when building.

Turns out the need for this package is already documented in
docs/postgres.rst.

docker/Dockerfile-dhvirtualenv

@@ -55,7 +55,8 @@ RUN apt-get update -qq -o Acquire::Languages=none \
         python3-pip \
         python3-setuptools \
         python3-venv \
-        sqlite3
+        sqlite3 \
+        libpq-dev
 
 COPY --from=builder /dh-virtualenv_1.1-1_all.deb /