Update contacts.py

forgot to add staticmethod, probably reason of failure in tests added it now

package/MDAnalysis/analysis/contacts.py

@@ -490,6 +490,7 @@ def _get_atomgroup(u, sel):
         else:
             raise TypeError(select_error_message)
 
+    @staticmethod
     def _get_box_func(ts, pbc):
         """Retrieve the dimensions of the simulation box based on PBC.