Deprecate bsddb for removal in Python 3.0.

Closes issue 3776.
Review by Nick Coghlan.
diff --git a/Lib/dbhash.py b/Lib/dbhash.py
index 9f8a9c3..5ca0bc1 100644
--- a/Lib/dbhash.py
+++ b/Lib/dbhash.py
@@ -1,6 +1,9 @@
 """Provide a (g)dbm-compatible interface to bsddb.hashopen."""
 
 import sys
+if sys.py3kwarning:
+    import warnings
+    warnings.warnpy3k("in 3.x, dbhash has been removed", DeprecationWarning, 2)
 try:
     import bsddb
 except ImportError: