Removed automap deprecation warning

Removed automap deprecation warning

Fixed a deprecation warning that was emitted when using
:func:`_automap.automap_base` without passing an existing
``Base``.

Fixes: #6529
<!-- Provide a general summary of your proposed changes in the Title field above -->

### Description
Fixed the automap's declarative import from sqlalchemy.orm instead from the deprecated ext.declarative package

### Checklist
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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.

**Have a nice day!**

Closes: #6533
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6533
Pull-request-sha: 46d40005eae51fd7423da66f6ad4072f65744c1c

Change-Id: I432df70d4221873ca229076d5d3ac20b2870c13b