Do not use translate to delete chars

In python3, str.translate() only takes one argument, we can not use
it to delete specific chars. Use other method instead.

Partially-Implements: blueprint magnum-python3

Change-Id: I3e1ab0969b9ef9bb8240195b8cb1496bd9bc2e4a

magnum/tests/unit/common/test_short_id.py

@@ -68,5 +68,6 @@ class ShortIdTest(testtools.TestCase):
 
         for id in ids:
             self.assertEqual(12, len(id))
-            self.assertFalse(id.translate(None, allowed_chars))
+            s = ''.join(ch for ch in id if ch not in allowed_chars)
+            self.assertEqual(s, '')
             self.assertEqual(1, ids.count(id))