Add public accessor `is_single_entity` to Query

Add public accessor `is_single_entity` to Query

Added accessor :attr:`.Query.is_single_entity` to :class:`.Query`, which
will indicate if the results returned by this :class:`.Query` will be a
list of ORM entities, or a tuple of entities or column expressions.
SQLAlchemy hopes to improve upon the behavior of single entity / tuples in
future releases such that the behavior would be explicit up front, however
this attribute should be helpful with the current behavior.  Pull request
courtesy Patrick Hayes.

Fixes: #4934
Closes: #4935
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4935
Pull-request-sha: 98f72b40a896761a65b048cc3722ff2dac4cf2b1

Change-Id: If5db5db3ea46a93406d76ef90b5b588149ba2986
(cherry picked from commit e55e749e182e257fcc60db383751d5afbe52ddfb)