Can't change schema via `Model._meta.schema` (regression from 2.X)
For now, can you try this change for 3.x?

```python
@contextmanager
def context(schema_name):
    with DB.atomic() as transaction:
        del DocumentsModel._meta.table  # Don't worry, this won't delete any data.
        DocumentsModel._meta.schema = schema_name
        yield transaction
        DocumentsModel._meta.schema = None
``` yes, this works for peewee==3.0.15 and fails on 2.X, thanks for suggestion Pushed 3.0.16, which contains fix for this issue. Thanks!
I'm working to work with several DB schemas and use `Model._meta.schema` to switch them, as shown in attached script. But for some reasons schema switch doesn't work in peewee 3.X. See console output. Please, suggest is it a bug or there is a better way to do it.

Thanks!

Console output

```
$ pip freeze | grep peewee
peewee==2.10.2
peewee-async==0.5.10
$ python test_schema_change.py
('SELECT "t1"."document_uuid", "t1"."name" FROM "user_1"."documents_metadata" AS t1 WHERE ("t1"."name" = %s)', ['test'])
('SELECT "t1"."document_uuid", "t1"."name" FROM "user_2"."documents_metadata" AS t1 WHERE ("t1"."name" = %s)', ['test'])
$ pip install peewee==3.0.15
Collecting peewee==3.0.15
Installing collected packages: peewee
  Found existing installation: peewee 2.10.2
    Uninstalling peewee-2.10.2:
      Successfully uninstalled peewee-2.10.2
Successfully installed peewee-3.0.15
$ python test_schema_change.py
('SELECT "t1"."document_uuid", "t1"."name" FROM "user_1"."documents_metadata" AS "t1" WHERE ("t1"."name" = %s)', ['test'])
('SELECT "t1"."document_uuid", "t1"."name" FROM "user_1"."documents_metadata" AS "t1" WHERE ("t1"."name" = %s)', ['test'])
```

Executed script

```
from contextlib import contextmanager
import peewee

DB = peewee.PostgresqlDatabase(
    "my_documents",
    user="username", password="top_secret"
)

class DocumentsModel(peewee.Model):

    uuid = peewee.UUIDField(primary_key=True, db_column="document_uuid")
    name = peewee.CharField(256)

    class Meta:
        db_table = "documents_metadata"
        database = DB


@contextmanager
def context(schema_name):
    with DB.atomic() as transaction:
        DocumentsModel._meta.schema = schema_name
        yield transaction
        DocumentsModel._meta.schema = None


with context('user_1'):
    print(DocumentsModel.filter(name="test").sql())

with context('user_2'):
    print(DocumentsModel.filter(name="test").sql())
```