Allow bytes in LVK injection format hdf

I don't know if the file specification changed, or if someone changed from py2 to py3 without thinking, but this attribute is bytes in some of the examples we have been given

bin/pycbc_convertinjfiletohdf

@@ -174,7 +174,7 @@ if args.injection_file.endswith(('.xml', '.xml.gz', '.xmlgz')):
 else:
     # Assume a HDF file - check if new LVK format first
     inj_file = h5py.File(args.injection_file, 'r')
-    if 'file_format' in inj_file.attrs:
+    if 'file_format' in inj_file.attrs or b'file_format' in inj_file.attrs:
         # Assume LVK as PyCBC doesn't have this attribute
         injclass = LVKNewStyleInjectionSet(args.injection_file)
     inj_file.close()