Couldn't open extension control file plpython3u.control : No such file or directory

Question

I want to have python3 in my postgresql database for writing stored procedures.

Being in the psql client, when I enter the command create extension plpython3u I get the error:

couldn't open extension control file /usr/share/postgresql/9.1/extension/plpython3u.control : No such file or directory

I have checked, there are plpythonu.control and plpython2u.control in the directory, but not the one for version 3.

However I have installed the packages python3 and python3-postgresql (among others) from depot. I'm using Ubuntu 12.04, kernel 3.2.0.38, with postgresql 9.1 installed.

What should I install (or do) to have the plpython3u.control file on my machine and have python3 available in my database?