fixes #4212 raise an error if selectable and flat/aliased are sent to with_polymorphi...

fixes #4212 raise an error if selectable and flat/aliased are sent to with_polymorphic simultaenously

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Hi,
This is a proposed fix for Issue #4212

When the selectable argument is passed to with_polymorphic it is already aliased. Therefore an argument error should be raised if flat is also True. I have added an if statement to the with_polymorphic function, that raises an Argument Error if selectable is none/False AND if flat=True. I have also included a test.

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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [X ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Kind Regards,

Ramon

Closes: #5232
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5232
Pull-request-sha: eae20cdadac82ca1e0352000fe0211e20b590e8b

Change-Id: I49c3022296608ede43de7f508fa993fa97dda81a