When using sqloxide, the logic for testing if a CTE is a select statement doesn't account for set operations.

How to reproduce the bug

SQLLab: WITH t AS ( SELECT 1 UNION ALL SELECT 2 ) SELECT * FROM t

Python interpreter:

import superset.sql_parse as sql_parse
assert sql_parse.sqloxide_parse is not None, "The case only fails when sqloxide is used"

parsed_query = sql_parse.ParsedQuery("""WITH t AS (
SELECT 1 UNION ALL SELECT 2
)
SELECT * FROM t""")

print("Is select:", parsed_query.is_select())

Expected results

Interpreter: Is select: True

Actual results

SQLLAb: Only SELECT statements are allowed against this database.
Interpreter: Is select: False

Environment

The problem arises only when sqloxide is installed.

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context