BUG #13861: Database dump does not load when both plpython2u and plpython3u functions are used

The following bug has been logged on the website:

Bug reference: 13861
Logged by: Roman Vanicek
Email address: roman(dot)vanicek(at)gmail(dot)com
PostgreSQL version: 9.5.0
Operating system: Linux Ubuntu 15.10
Description:

Use case:
I have a database that uses both plpython2u and plpython3u extensions and
some functions using them.

When trying do restore the database on 9.5 server after dump, it produces
fatal error.

On postgresql-9.4:
pg_dumpall > all.dump

On postgresql-9.5:
psql < all.dump

Result:
FATAL: Python major version mismatch in session
DETAIL: This session has previously used Python major version 2, and it is
now attempting to use Python major version 3.
HINT: Start a new session to use a different Python major version.
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
connection to server was lost