Using `in_` with a subquery applies a table alias to the subquery
That is a kind of weird construction...typically wouldn't you write instead:

```python
subquery = Team.select().where(Team.id == 1)
query = User.select().where(User.team.in_(subquery))
``` In this minimal example, perhaps. The actual use of this construction will of course be more complex (both the primary and subquery) and I need the boolean result of the `in` statement alongside other data. Fixed, thanks for reporting. Awesome! Thanks for the fast fix.
When using `in_` with a subquery, a table alias is applied to the subquery in the final SQL statement which results in an invalid SQL statement.

F.e. the sample below produces the following SQL statement. Notice the `AS "t3"` part.
```sql
SELECT ("t1"."team_id" IN (SELECT "t2"."id" FROM "team" AS "t2" WHERE ("t2"."id" = 1)) AS "t3") FROM "user" AS "t1" LIMIT 1 OFFSET 0
```

Minimal example:

```python
import logging
logging.basicConfig(level=logging.DEBUG)
from peewee import *

database = PostgresqlDatabase('scoreboard')

class Base(Model):
    class Meta:
        database = database

class Team(Base):
    name = TextField(unique=True)
    
class User(Base):
    username = TextField(unique=True)
    team = ForeignKeyField(Team, null=True, backref='members')

subquery = Team.select().where(Team.id == 1)
User.select(User.team_id.in_(subquery)).get()
```

This yields the following output:
```
DEBUG:peewee:('SELECT ("t1"."team_id" IN (SELECT "t2"."id" FROM "team" AS "t2" WHERE ("t2"."id" = %s)) AS "t3") FROM "user" AS "t1" LIMIT %s OFFSET %s', [1, 1, 0])
Traceback (most recent call last):
  File "/home/ingmar/.virtualenvs/scoreboard-api-a2XBUqg6/lib/python3.7/site-packages/peewee.py", line 2653, in execute_sql
    cursor.execute(sql, params or ())
psycopg2.ProgrammingError: syntax error at or near "AS"
LINE 1: ..."."id" FROM "team" AS "t2" WHERE ("t2"."id" = 1)) AS "t3") F...
```