Must be superuser to create this extension.

test_dd=> CREATE EXTENSION plpython3u;
ERROR: permission denied to create extension "plpython3u"
HINT: Must be superuser to create this extension.
test_dd=> CREATE EXTENSION plperl;
CREATE EXTENSION

Does that mean that installing plpython3u can break security model on
shared database server? Does that mean that plperl won't break it?
Security implications (on shared database server) of intalling
extensions are still undocumented.