Move pk on single-inh subclass check below conflict resolution check

Move pk on single-inh subclass check below conflict resolution check

The column conflict resolution technique discussed at
:ref:`declarative_column_conflicts` is now functional for a :class:`.Column`
that is also a primary key column.  Previously, a check for primary key
columns declared on a single-inheritance subclass would occur before the
column copy were allowed to pass.

Fixes: #4352
Change-Id: Id4c025da53c28e58db6b549fe398f25f8a90d355
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/483
(cherry picked from commit 3ed79a5c18c14d842280805d1dae8a9c99ec8f18)