select 'alter user ' || username || ' identified by values ''' || password || ''';' from dba_users;

To avoid the usually useful output in this case I used to set a few options, such as in:

set heading off pages 0 trimspool on lines 120 feedback off echo off termout off

Inside a shell script this could look as follows:

$ORACLE_HOME/bin/sqlplus  / as sysdba <<EOF
set heading off pages 0 trimspool on lines 120 feedback off echo off termout off
spool /tmp/users.sql
select 'alter user ' || username || ' identified by values ''' || password || ''';' from dba_users;
exit
EOF