Allow to use peewee.SQL from `from_`
I don't really like this solution. The join will not correctly set-up the model graph, so the best solution in my opinion is to write:

```python
q = User.select(User, Tweet).from_(SQL(f'`users` as `t1` USE INDEX(`user_id`) ')).join(Tweet)
for x in q.objects():
   # assume username is on User table, and content is on Tweet table.
   print(x.username, x.content)
```
**Problem:**
Tried to apply custom SQL to `from_` function from this example https://github.com/coleifer/peewee/issues/1289#issuecomment-307128658 However in my query, I use some joins.

Here is example
```python
q = User.select(User, Tweet).from_(SQL(f'`users` as `t1` USE INDEX(`user_id`) ')).join(Tweet)
for x in q:
   print(x)
```

But without fix, exception is thrown
```
Traceback (most recent call last):
  File "JetBrains\PyCharm2021.2\scratches\scratch_71.py", line 27, in <module>
    for file in q:
  File "venv\lib\site-packages\peewee.py", line 4408, in next
    self.cursor_wrapper.iterate()
  File "venv\lib\site-packages\peewee.py", line 4325, in iterate
    self.initialize()  # Lazy initialization.
  File "venv\lib\site-packages\peewee.py", line 7587, in initialize
    if curr not in self.joins:
TypeError: unhashable type: 'SQL'
```

**Solution**
When initializing ModelCursorWrapper skip SQL instances