insert_from with raw SQL query broke between Peewee 2 and 3
It looks like it's entering endless recursion...will take a look. Fixed, thanks for reporting. Thanks for the fast fix! Thanks for the quality bug report.
I've been using `insert_from` to migrate data from a non-Peewee table to a Peewee created table. It works as intended with Peewee 2, I'm moving to 3 but it hangs and keeps increasing memory usage.

Below is a minimal, full working example showing the issue.
```
from peewee import SqliteDatabase, Model, CharField, SQL

database = SqliteDatabase(":memory:")

class Foo(Model):
    foo = CharField()

    class Meta:
        database = database

database.create_tables([Foo])
database.execute_sql("CREATE TABLE Bar (bar TEXT);")
database.execute_sql("INSERT INTO Bar (bar) VALUES ('testvalue');")

cursor = database.execute_sql("SELECT * FROM Bar;")
print(cursor.fetchone())

# Works correctly with Peewee 2.10.2, hangs and memory leak with 3.5.0
Foo.insert_from(query=SQL("SELECT bar FROM Bar;"), fields=[Foo.foo]).execute()
print(list(Foo.select()))
```