pg_upgrade: language "plpythonu" is not trusted

Question

I'm trying to perform PostgreSQL 9.6 -> 10 upgrade with link method:

/usr/lib/postgresql/10/bin/pg_upgrade --link \
 --old-datadir=/var/lib/postgresql/9.6/main \
 --new-datadir=/var/lib/postgresql/10/main \
 --old-bindir=/usr/lib/postgresql/9.6/bin \
 --new-bindir=/usr/lib/postgresql/10/bin \
 --old-options=' -c config_file=/etc/postgresql/9.6/main/postgresql.conf' \
 --new-options=' -c config_file=/etc/postgresql/10/main/postgresql.conf' \
 --old-port=5432 \
 --new-port=5433

however the upgrade process always fails on "untrusted languages":

pg_restore: [archiver (db)] Error while PROCESSING TOC:
pg_restore: [archiver (db)] Error from TOC entry 535968; 0 0 ACL LANGUAGE "plpythonu" postgres
pg_restore: [archiver (db)] could not execute query: ERROR:  language "plpythonu" is not trusted
DETAIL:  GRANT and REVOKE are not allowed on untrusted languages, because only superusers can use untrusted languages.
    Command was: GRANT ALL ON LANGUAGE "plpythonu" TO "mydb";

Any idea how to fix this? I've already tried dropping plpythonu extension before upgrade.