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

pygmx/tpxio.pyx

@@ -8,10 +8,10 @@ import numpy as np
 import cython
 cimport numpy as np
 
-from utility cimport *
-from math cimport *
-from mdtypes cimport *
-from topology cimport *
+from .utility cimport *
+from .math cimport *
+from .mdtypes cimport *
+from .topology cimport *
 
 
 cdef extern from "gromacs/fileio/tpxio.h":