Drop Constraint Syntax Error in MySQL 8.0.18
For MySQL you would use the `drop_foreign_key_constraint` migrator method. I couldn't find it in peewee's documentation. Maybe could add it later. Thanks! Oh, it's spent a lot of time to find `drop_foreign_key_constraint` method... It will be very usefull to add it to the docs. The foreign-key constraint should be dropped automatically when dropping a foreign-key column.
Hi,

I'm using `peewee.playhouse.migrate.drop_constraint` to do some migrations on MySQL 8.0.18.
If failed with error message: 
<pre>
`Error in query (1064): Syntax error near 'CONSTRAINT `constraint_name`' at line 1`
</pre>

I've checked the SQL.
This will fail: <pre>ALTER TABLE xxx DROP CONSTRAINTS `XXX`</pre>
This will succeed: <pre>ALTER TABLE xxx DROP FOREIGN KEY `XXX`</pre>

Is there anything peewee can improve? Or configurations in MySQL I ignored?

Thanks for your help!