Fix: ensure Expression is not an iterable to avoid inf. loops

[georgesittas]

Before:

>>> import sqlglot
>>> for x in sqlglot.parse_one("select 1"):
...     pass
...
<infinite loop - hangs>

After:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sqlglot/expressions.py", line 764, in __iter__
    raise TypeError(f"'{self.__class__.__name__}' object is not iterable")
TypeError: 'Select' object is not iterable

[georgesittas]

Not sure how expensive this import can be; it was the simplest solution, but perhaps a try-except block where we call iter(value) is faster / better?

[tobymao]

nah it's fine

[georgesittas]

cool