Access denied for user 'debian-sys-maint' - installing mysql?

A bit late but here it is: Firstly I'm on

Distributor ID: Ubuntu
Description:    Ubuntu 16.10
Release:    16.10
Codename:   yakkety

I had to "uncheck" some software repositories. It can be easily done on Software & Updates GUI on the Other Software tab. I unchecked:

• unstable repos
• xenial repos
• "disabled on upgrade to yakkety" repositories

The only checked repositories for me were:

software&updates window

Then, I solved the problem this way:

sudo su

root@1w3j: mysql -u root -p
Enter password:
mysql> GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY '<your password>';
ERROR 1819 (HY000): Your password does not satisfy the current policy requirements

If the error(1819) is raised, type this on the mysql terminal

mysql> uninstall plugin validate_password;

Then restart mysql: systemctl restart mysql

Finally

apt install -f

to fix broken dependencies

If error continues, enter again to mysql terminal, login: type this:

mysql> GRANT ALL PRIVILEGES ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY '<your password>'

apt -f install for the last time.

Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libmecab2
Use 'sudo apt autoremove' to remove it.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
2 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Setting up mysql-server-5.7 (5.7.17-0ubuntu0.16.10.1) ...
Checking if update is needed.
Checking server version.
Running queries to upgrade MySQL server.
Checking system database.
mysql.columns_priv                                 OK
mysql.db                                           OK
mysql.engine_cost                                  OK
mysql.event                                        OK
mysql.func                                         OK
mysql.general_log                                  OK
mysql.gtid_executed                                OK
mysql.help_category                                OK
mysql.help_keyword                                 OK
mysql.help_relation                                OK
mysql.help_topic                                   OK
mysql.innodb_index_stats                           OK
mysql.innodb_table_stats                           OK
mysql.ndb_binlog_index                             OK
mysql.plugin                                       OK
mysql.proc                                         OK
mysql.procs_priv                                   OK
mysql.proxies_priv                                 OK
mysql.server_cost                                  OK
mysql.servers                                      OK
mysql.slave_master_info                            OK
mysql.slave_relay_log_info                         OK
mysql.slave_worker_info                            OK
mysql.slow_log                                     OK
mysql.tables_priv                                  OK
mysql.time_zone                                    OK
mysql.time_zone_leap_second                        OK
mysql.time_zone_name                               OK
mysql.time_zone_transition                         OK
mysql.time_zone_transition_type                    OK
mysql.user                                         OK
The sys schema is already up to date (version 1.5.1).
Checking databases.

(your databases will be shown here...)

Upgrade process completed successfully.
Checking if update is needed.
Setting up mysql-server (5.7.17-0ubuntu0.16.10.1) ...

Other way to solve was: apt -u dist-upgrade