erp5_access_token: use hmac.compare_digest instead of string comparison

in order to avoid timing attacks

/reviewed-on !115

bt5/erp5_access_token/SkinTemplateItem/portal_skins/erp5_access_token/RestrictedAccessToken_getExternalLogin.py

 from zExceptions import Unauthorized
+import hmac
 if REQUEST is not None:
   raise Unauthorized
 
@@ -15,7 +16,8 @@ if access_token_document.getValidationState() == 'validated':
     if reference is None:
       reference = request.form.get("access_token_secret", "INVALID_REFERERENCE")
 
-    if access_token_document.getReference() != reference:
+    # use hmac.compare_digest and not string comparison to avoid timing attacks
+    if not hmac.compare_digest(access_token_document.getReference(), reference):
       return None
     
     agent_document = access_token_document.getAgentValue()