Fix Metaclass usage for Python 3

[bpgoldsb]

Fix the way SqlAlchemyItemMeta is used in SqlItem to be compatible with Python 2.x and 3.x.

[philtweir]

For anybody else having this issue, a workaround until this PR can be merged is to import SqlAlchemyItemMeta from scrapy_sqlitem.sqlitem and use, e.g.

class MyItem(SqlItem, metaclass=SqlAlchemyItemMeta)`:
     ...