Operator does not exist: json = json?
Sorry, I just found #2504. Closing.
👋🏻  thanks for peewee, it's awesome.

I'm having troubles with `playhouse.postgres_ext.JSONField`. The minimum reproducible code is:
```python
from playhouse.postgres_ext import *
database = PostgresqlExtDatabase(...)

class DataProduct(Model):
    class Meta:
        database = database
    
    id = AutoField()
    
    release = TextField()
    filetype = TextField()
    kwargs = JSONField()

DataProduct.create_table()

dp, created = DataProduct.get_or_create(
    release="new",
    filetype="some",
    kwargs={"version": 3.14}
)
```

This raises the following exception:
```
.../peewee.py in execute_sql(self, sql, params, commit)
   3142             cursor = self.cursor(commit)
   3143             try:
-> 3144                 cursor.execute(sql, params or ())
   3145             except Exception:
   3146                 if self.autorollback and not self.in_transaction():

ProgrammingError: operator does not exist: json = json
LINE 1: ...ND ("t1"."filetype" = 'some')) AND ("t1"."kwargs" = CAST('{"...
                                                             ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.
```

I have the following relevant versions:
- `peewee`: 3.14.4
- `psycopg2`: 2.9.1
- PostgreSQL: 12.2

What am I doing wrong here?

Edit: the equivalent code works fine when I am using `playhouse.sqlite_ext.SqliteExtDatabase`.

