Couldn't open extension control file plpython3u.control : No such file or directory
The package to install is postgresql-plpython3.

By way of dependencies, this will install postgresql-plpython3-9.1, which provides these files:


$ dpkg -L postgresql-plpython3-9.1
/.
/usr
/usr/share
/usr/share/doc
/usr/share/doc/postgresql-plpython3-9.1
/usr/share/doc/postgresql-plpython3-9.1/copyright
/usr/share/postgresql
/usr/share/postgresql/9.1
/usr/share/postgresql/9.1/extension
/usr/share/postgresql/9.1/extension/plpython3u--1.0.sql
/usr/share/postgresql/9.1/extension/plpython3u.control
/usr/share/postgresql/9.1/extension/plpython3u--unpackaged--1.0.sql
/usr/lib
/usr/lib/postgresql
/usr/lib/postgresql/9.1
/usr/lib/postgresql/9.1/lib
/usr/lib/postgresql/9.1/lib/plpython3.so
/usr/share/doc/postgresql-plpython3-9.1/changelog.Debian.gz


and then you can do:

postgres=# create extension plpython3u;
CREATE EXTENSION