use column.copy() for column operations due to SQLAlchemy "column is already attached" issue

[zzzeek]

the change in sqlalchemy/sqlalchemy#5618 is now hitting me as I'm trying to write a recipe for someone. when we take columns into the operations we should just run _copy() on them so that we dont get the already attached to table issue.

[sqla-tester]

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

Copy column that's already attached https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/2345