typing: annotate Exists.select() to return Select[bool]

typing: annotate Exists.select() to return Select[bool]

Fixes: #11231
A query of the form:

``` sql
SELECT EXISTS (
    SELECT 1
    FROM ...
    WHERE ...
)
```

… returns a boolean.

Closes: #11233
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11233
Pull-request-sha: 1bec1cac731eb42e097948f84ae3d0ef133f8a9a

Change-Id: I407a3bd9ed21a180c6c3ff02250aa0a9fbe502d7