Update how private keys are handled when signing data

This change updates how private key bytes are loaded when signing
data. The prior approach required binascii to unhexlify the byte
string. The current approach removes this requirement, matching
the rest of the library. All unit tests have been updated to
reflect this change.

kmip/services/server/crypto/engine.py

@@ -13,7 +13,6 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import binascii
 import logging
 import os
 
@@ -1204,14 +1203,14 @@ class CryptographyEngine(api.CryptographicEngine):
 
         try:
             private_key = serialization.load_pem_private_key(
-                binascii.unhexlify(bytes),
+                bytes,
                 password=None,
                 backend=default_backend()
             )
             return private_key
         except:
             private_key = serialization.load_der_private_key(
-                binascii.unhexlify(bytes),
+                bytes,
                 password=None,
                 backend=default_backend()
             )