fixed relative imports (language_level=2), need to be explicit for python3 (language_level=3)

pygmx/xtcio.pyx

@@ -9,9 +9,9 @@ import numpy as np
 cimport numpy as np
 import os
 
-from utility cimport *
-from math cimport *
-from fileio cimport *
+from .utility cimport *
+from .math cimport *
+from .fileio cimport *
 from .gromacs.reader import INDEX_MAGIC, SubscriptableReader, XTCFrame
 from .errors import InvalidIndexException, InvalidMagicException, XTCError
 
@@ -288,4 +288,4 @@ def append_xtcfile(filename, step, time, box, coords, prec):
 
 
 
-    
+