private constructor elements on CreateTableOp make it impossible to copy w/ public API

[zzzeek]

Discussed in #1192

private attributes in 3b09a89 and 6aad686 seem to be breaking the recipe at https://alembic.sqlalchemy.org/en/latest/cookbook.html#apply-custom-sorting-to-table-columns-within-create-table ; at least _namespace_metadata seems to be required as illustrated at #1192.

see if these private attributes can somehow be set up so that the recipe works again without these being named