BUG: fix read_sql delegation for queries without select statement (GH7324)

The select check was introduced for mysql legacy compatibility, but introduced problem if the query did not contain a select statement (eg a stored procedure)
Merge pull request #7326 from jorisvandenbossche/sql-delegation

BUG: fix read_sql delegation for queries without select statement (GH7324)
BUG: Fix error when reading postgres table with timezone #7139

Fixes an issue where read_sql_table() will throw an error if it is
reading a postgres table with `timestamp with time zone` fields that
contain entries with different time zones (such as for DST).

This also adds a new keyword, convert_dates_to_utc, which optionally
allows the caller to convert `timestamp with time zone` datetimes into
UTC, which allows pandas to store them internally as
`numpy.datetime64`s, allowing for more efficient storage and operation
speed on these columns.