RFC: `fetchdictarray` API change

[peytondmurray]

Currently fetchdictarray accepts three arguments:

size : int, optional
    The number of rows to fetch. Use -1 (the default) to fetch all
    remaining rows.
return_nulls : boolean, optional
    If True, information about null values will be included adding a
    boolean array using as key a string  built by concatenating the
    column name and null_suffix.
null_suffix : string, optional
    A string used as a suffix when building the key for null values.
    Only used if return_nulls is True.

I propose removing return_nulls; if null_suffix == None, nulls are discarded. This would simplify code on the C side and the Python side, at the cost of backward compatibility.