Remove dependency on h5py 2.10.0

We had acquired a dependency on h5py>=2.10.0 when we introduced variable-length UTF8 string attributes (specifically, we used h5py.string_dtype). This was sad, because scanpy has a dependency h5py!=2.10.0 which makes loompy and scanpy strictly incompatible. However, it turns out there's a way to describe the same thing that works in older versions of h5py (specifically, using special_dtype(vlen=str)). This release uses the old way, and removes the dependency on h5py 2.10.0.