Update deprecation warning for diffpy.Structure module

Author: sbillinge

src/diffpy/structure/__init__.py

@@ -59,7 +59,7 @@ def __getattr__(self, name):
         import warnings
 
         warnings.warn(
-            "Module 'diffpy.Structure' is deprecated and will be removed in version 3.8. "
+            "Module 'diffpy.Structure' is deprecated and will be removed in version 4.0. "
             "Use 'diffpy.structure' instead.",
             DeprecationWarning,
             stacklevel=2,