MySQL does not support the specification of where clauses...
Hi, I read the issue https://github.com/coleifer/peewee/issues/1307 and know I can use upsert-like method `on_conflict`, however when I try I got error: `MySQL does not support the specification of where clauses or conflict targets for conflict resolution.` , anyone knows why?

```
res = (Emp
       .insert(first='foo', empno='125')
       .on_conflict(
            conflict_target=(Emp.empno,),
            preserve=(Emp.first, Emp.last),
            update={Emp.empno: '125.1'})
        .execute())
```