Remove invalid escape sequences

A recent style update to Python 3.6 adds deprecation W605, which
tightens the usage of invalid escape sequences. This patch removes
any instances of invalid escape sequences from the PyKMIP code
base, bringing the library back up to compliance with Python style.

kmip/tests/unit/services/server/test_engine.py

@@ -4108,7 +4108,7 @@ class TestKmipEngine(testtools.TestCase):
         self.assertRaisesRegexp(
             exceptions.InvalidField,
             "The cryptographic length must correspond to a valid number of "
-            "bytes \(i.e., it must be a multiple of 8\).",
+            "bytes; it must be a multiple of 8.",
             e._process_derive_key,
             *args
         )