add missing stacklevel in a Python warning

Author: neutrinoceros

pytools/vtk_io.py

@@ -99,7 +99,7 @@ def _load_header(self, fh, geometry=None):
                     native_name, _ncomp, native_dim, _dtype = d.split()
                     self.native_coordinates[native_name] = np.fromfile(fh, dtype=dt, count=int(native_dim))
                 else:
-                    warnings.warn("Found unknown field %s" % d)
+                    warnings.warn("Found unknown field %s" % d, stacklevel=3)
                 fh.readline()  # skip extra linefeed (empty line)
 
         if self.geometry is None: