Fix plane count check in bandmask (had always passed).

[JustinTArthur]

This test was always passing because it was creating a single-element list every time. I'm assuming the intention was to finalize an iterable if one was used instead of a sequence, so I maintained the conversion.

[quietvoid]

bandmask only supports a single plane index, so the check itself doesn't make sense.

[JustinTArthur]

I think that's what the check is trying to enforce.

[quietvoid]

The function expects an int already, users shouldn't be passing a list.

[quietvoid]

I just removed the check in 0951c33.