PostgresqlDatabase class does not have `_upsert` statement
Follow-up, Postgresql has implemented `UPSERT` in version > 9.5. [See here](https://wiki.postgresql.org/wiki/UPSERT)
 @cheetah90 -- there are two issues currently open tracking this exact thing:
- #942 
- #853

Looking at the [PostgresqlDatabase class](https://github.com/coleifer/peewee/blob/b84ca0c8bc724b8cccd0b5bbd557abc04885c478/peewee.py#L3945), it does not specify the statement for `_upsert`, so when [`query._upsert` is called here](https://github.com/coleifer/peewee/blob/b84ca0c8bc724b8cccd0b5bbd557abc04885c478/peewee.py#L1982) , it just return `None`.
