fix: only use covariant Row type variable

fix: only use covariant Row type variable

The real variance of Row type variable is covariant, per definition of
RowMaker (RowMaker[B] is a subtype of RowMaker[A] if B is a subtype of
A; similar to the Box type in [mypy documentation][]).

By dropping the Row type variable from Cursor argument in RowFactory, we
are now able to only use the covariant Row variable (previously Row_co,
now Row). Indeed, RowFactory does not actually depend on Cursor on being
parametrized on Row; rather it's the other way around (Cursor's Row type
variable comes from its row factory).

[mypy documentation]: https://mypy.readthedocs.io/en/latest/generics.html#variance-of-generic-types