Switch to our fork of dh-virtualenv for compatibility with Python 3.11

changelog.d/14774.misc

@@ -0,0 +1 @@
+Switch to our fork of `dh-virtualenv` to work around an upstream Python 3.11 incompatibility.

docker/Dockerfile-dhvirtualenv

@@ -36,8 +36,10 @@ RUN env DEBIAN_FRONTEND=noninteractive apt-get install \
         wget
 
 # fetch and unpack the package
+# We are temporarily using a fork of dh-virtualenv due to an incompatibility with Python 3.11, which ships with
+# Debian sid. TODO: Switch back to upstream once https://github.com/spotify/dh-virtualenv/pull/354 has merged.
 RUN mkdir /dh-virtualenv
-RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/spotify/dh-virtualenv/archive/refs/tags/1.2.2.tar.gz
+RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/matrix-org/dh-virtualenv/archive/refs/tags/matrixorg-2023010302.tar.gz
 RUN tar -xv --strip-components=1 -C /dh-virtualenv -f /dh-virtualenv.tar.gz
 
 # install its build deps. We do another apt-cache-update here, because we might