This commit fixes an issue in the dask cursor that would cause it to
return `None` instead of `0` when performing a COUNT() query on Windows.
`dask-sql` returns an empty data frame when there are no matches. This
is normally translated to 0 in the DaskCursor class when using fetchone
with a numeric result.

Unfortunately the logic to detect a numeric result did not behave
correctly on Windows. The dtype used is int64, which on linux appears to
compare equal to int but does not compare equal on Windows.

This commit uses `np.issubdtype` to have a more robust check for integer
types.