Master-master replication with PostgreSQL

Hello

We're looking for an open-source database solution that has a Python
interface and will do master-master replication. My client has a flawed MS
SQL Server system, with the replication hand-coded in Python. They don't
want to pay very high licence fees for the MS SQL Server Enterprise version
at 20 sites across the UK.

There is one central server with 19 branches. Some tables need to replicate
from the central server to the branches. Other tables are centralised from
the branches into one totalling table at the centre. A few tables need to
replicate in both directions.

From what I've read, Slony-I does only master-slave replication and Slony-II
is not being actively developed. Is this right? Are there any viable
master-master replication tools for PostgreSQL. (They could be
commercial/paid for if necessary.)

Rob