Fix Debian builds on Xenial

Adds note about updating dh-virtualenv once we drop support for Xenial.

We can't update now, because it needs debhelper 12, while Xenial only
backports 10.

Signed-off-by: Dan Callahan <danc@element.io>

changelog.d/9254.misc

@@ -0,0 +1 @@
+Fix Debian package building on Ubuntu 16.04 LTS (Xenial).

debian/build_virtualenv

@@ -33,11 +33,13 @@ esac
 # Use --builtin-venv to use the better `venv` module from CPython 3.4+ rather
 # than the 2/3 compatible `virtualenv`.
 
+# Pin pip to 20.3.4 to fix breakage in 21.0 on py3.5 (xenial)
+
 dh_virtualenv \
     --install-suffix "matrix-synapse" \
     --builtin-venv \
     --python "$SNAKE" \
-    --upgrade-pip \
+    --upgrade-pip-to="20.3.4" \
     --preinstall="lxml" \
     --preinstall="mock" \
     --extra-pip-arg="--no-cache-dir" \

debian/changelog

@@ -1,3 +1,9 @@
+matrix-synapse-py3 (1.26.0+nmu1) UNRELEASED; urgency=medium
+
+  * Fix build on Ubuntu 16.04 LTS (Xenial).
+
+ -- Dan Callahan <danc@element.io>  Thu, 28 Jan 2021 16:21:03 +0000
+
 matrix-synapse-py3 (1.26.0) stable; urgency=medium
 
   [ Richard van der Hoff ]

docker/Dockerfile-dhvirtualenv

@@ -27,6 +27,7 @@ RUN env DEBIAN_FRONTEND=noninteractive apt-get install \
         wget
 
 # fetch and unpack the package
+# TODO: Upgrade to 1.2.2 once xenial is dropped
 RUN mkdir /dh-virtualenv
 RUN wget -q -O /dh-virtualenv.tar.gz https://github.com/spotify/dh-virtualenv/archive/ac6e1b1.tar.gz
 RUN tar -xv --strip-components=1 -C /dh-virtualenv -f /dh-virtualenv.tar.gz