Update the ssencrypt logic for python3.

We updated the underlying methods to always be passing unicode strings
aronud and only encode them to bytes when we need to hash them.

lms/djangoapps/verify_student/ssencrypt.py

@@ -175,8 +175,8 @@ def generate_signed_message(method, headers_dict, body_dict, access_key, secret_
     message = signing_format_message(method, headers_dict, body_dict)
 
     # hmac needs a byte string for it's starting key, can't be unicode.
-    hashed = hmac.new(secret_key.encode('utf-8'), message, sha256)
-    signature = binascii.b2a_base64(hashed.digest()).rstrip('\n')
+    hashed = hmac.new(secret_key.encode('utf-8'), message.encode('utf-8'), sha256)
+    signature = binascii.b2a_base64(hashed.digest()).rstrip(b'\n')
     authorization_header = u"SSI {}:{}".format(access_key, signature)
 
     message += '\n'
@@ -223,12 +223,12 @@ def body_string(body_dict, prefix=""):
                 if isinstance(arr, dict):
                     body_list.append(body_string(arr, u"{}.{}.".format(key, i)))
                 else:
-                    body_list.append(u"{}.{}:{}\n".format(key, i, arr).encode('utf-8'))
+                    body_list.append(u"{}.{}:{}\n".format(key, i, arr))
         elif isinstance(value, dict):
             body_list.append(body_string(value, key + ":"))
         else:
             if value is None:
                 value = "null"
-            body_list.append(u"{}{}:{}\n".format(prefix, key, value).encode('utf-8'))
+            body_list.append(u"{}{}:{}\n".format(prefix, key, value))
 
     return "".join(body_list)  # Note that trailing \n's are important