matrix-synapse: python3-psycopg2 from backports
matrix-synapse 1.26 fails to start on FreedomBox stable if it doesn't find the right version of python3-psycopg2. Version 2.8 of this library is currently not available in Debian stable and has to be installed from the buster-backports repository.
Tested the upgrade on a FreedomBox stable virtual machine.
Signed-off-by: Joseph Nuthalapati njoseph@riseup.net