made dbshelve test conditional on bsddb3 presence

nospam/tests/test_dbshelve.py

+import os
 import shutil
 import tempfile
 import unittest
-from bsddb3 import db
+
+# Make the test conditional on successful bsddb3 import.
+try:
+    from bsddb3 import db
+except ImportError:
+    from nose.exc import SkipTest
+    raise SkipTest('LevelDB not found')
+
 from nospam.dbshelve import dbshelve
 from nospam.dbshelve import connect_db
-import os
+
 
 class TestDbshelve(unittest.TestCase):