DType parent class / type alias

[MarcoGorelli]

Also splitting this off from #187 (comment)

We have various dtypes: Int64, Int32, ...., Bool, which are part of the API.

We have functions which accept any dtype, such as

def column_from_sequence(sequence: Sequence[Scalar], *, dtype) -> Column:

How do we type dtype? I originally wanted to have DType be a common superclass of the dtypes, so we could have

def column_from_sequence(sequence: Sequence[Scalar], *, dtype: DType) -> Column:

but it's been brought up that this isn't desirable because it would add a class to the namespace for the sole purpose of type checking.

Are there alternatives which would be acceptable?