1234.5679 is the special code sent by the client to request SSL-encrypted database connections, and support for that has been in PostgreSQL since commit e0e7daef6da in 1999. But your PostgreSQL cannot be that old, because support for protocol version 3.0 was not added before 2003.
Actually, from studying src/backend/postmaster/postmaster.c and reading the mailing list, this is a bug on the PostgreSQL server:
The client must be configured to try GSS authentication, and when the server rejects, it wants to negotiate an SSL connections, but the server doesn’t expect that at this point; hence the error.
See the discussion here. The bug has been fixed with release 12.3.
As a workaround, disable either GSS authentication or SSL negotiation on the client.
In psycopg2, disabling SSL is done by using sslmode="disable" in the connection string, and disabling GSS is done with gssencmode="disable". See the documentation for details.