matrix-synapse: Error installing application

Matrix Synapse installation fails on Debian Buster Error installing application: Error during installation E: Unable to correct problems, you have held broken packages.

I tried to install matrix-synapse manually, error:

The following packages have unmet dependencies:
 matrix-synapse : Depends: python3-typing-extensions (>= 3.7.4) but 3.7.2-1 is to be installed
                  Recommends: python3-psycopg2 (>= 2.6) but it is not going to be installed

On another server, I see that matrix-synapse upgrades are held back (package was already installed):

# apt list --installed  matrix-synapse 
Listing... Done
matrix-synapse/now 1.3.0-1~bpo10+1 all [installed,upgradable to: 1.5.1-1~bpo10+1

It is nice that this issue doesn't block other package installations/upgrades.