[4.0.0] deprecate mysqlclient

[markuman]

There is an active poll: #645
Please participate if you like.

[markuman]

Let's close the vote tomorror.
Active contributors were hilighted and it was also posted in the ansible forum.
Furthermore the result is clear.

[Andersson007]

@markuman sounds good, thanks!

[markuman]

The poll is closed now. The result is clear.

The next major release 4.0.0 should mark the support for mysqlclient as deprecated and 5.0.0 should remove it.