caching_sha2_password authentication plugin support (MySQL-8.0)

[grooverdan]

PyMySQL doesn't implement the default MySQL-8.0 authentication protocol

https://dev.mysql.com/doc/dev/mysql-server/latest/page_caching_sha2_authentication_exchanges.html

Will require fix for #532 as basis for some paths of the implementation.

[LecoLu]

I also encountered this problem and hope to solve it as soon as possible。

[moxuec]

不支持mysql8.0 期待作者的更新

[zzzeek]

as a workaround, recommend folks use mysql_native_password (https://dev.mysql.com/doc/refman/8.0/en/native-pluggable-authentication.html) :

[mysqld]
# brand new server before any users are created
default-authentication-plugin=mysql_native_password

it's good enough for mariadb so it's good enough for Oracle