unique/index flag broken by acc82bf

[zzzeek]

the create_table we changed in #844 is also used by the op.create_table() op where we don't have a reflected table to work with so the check we added must be made more specific still.

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

differentiate CreateTableOp from model vs. op.create_table() https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/2839