Add Non linear CTE support

Add Non linear CTE support

"Compound select" methods like :meth:`_sql.Select.union`,
:meth:`_sql.Select.intersect_all` etc. now accept ``*other`` as an argument
rather than ``other`` to allow for multiple additional SELECTs to be
compounded with the parent statement at once. In particular, the change as
applied to :meth:`_sql.CTE.union` and :meth:`_sql.CTE.union_all` now allow
for a so-called "non-linear CTE" to be created with the :class:`_sql.CTE`
construct, whereas previously there was no way to have more than two CTE
sub-elements in a UNION together while still correctly calling upon the CTE
in recursive fashion. Pull request courtesy Eric Masseran.

Allow:

```sql
WITH RECURSIVE nodes(x) AS (
   SELECT 59
   UNION
   SELECT aa FROM edge JOIN nodes ON bb=x
   UNION
   SELECT bb FROM edge JOIN nodes ON aa=x
)
SELECT x FROM nodes;
```

Based on @zzzeek suggestion: https://github.com/sqlalchemy/sqlalchemy/pull/7133#issuecomment-933882348

Fixes: #7259
Closes: #7260
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7260
Pull-request-sha: 2565a5fd4b1940e92125e53aeaa731cc682f49bb

Change-Id: I685c8379762b5fb6ab4107ff8f4d8a4de70c0ca6
(cherry picked from commit 958f902b1fc528fed0be550bc573545de47ed854)