[SDK] Fix CLI authentication (#2986)

* fix cli auth

* fix

sdk/python/kfp/_auth.py

@@ -37,7 +37,8 @@ def get_gcp_access_token():
     https://cloud.google.com/sdk/gcloud/reference/auth/application-default/print-access-token
     """
     args = ['gcloud', 'auth', 'print-access-token']
-    return subprocess.check_output(args).rstrip()
+    # Casting to string to accommodate API server request schema.
+    return subprocess.check_output(args).rstrip().decode("utf-8")
 
 def get_auth_token(client_id, other_client_id, other_client_secret):
     """Gets auth token from default service account or user account."""