[Python3] Fix test failures in update_checkout (#33141)

Use correct relative import for Python 2&3

Remove unused variable

Fix import ordering per python_lint

Author: tbkka

utils/update_checkout/tests/test_update_worktree.py

@@ -12,9 +12,9 @@
 
 import os
 
-from scheme_mock import call_quietly
-
 from . import scheme_mock
+from .scheme_mock import call_quietly
+
 
 WORKTREE_NAME = "feature"
 

utils/update_checkout/update_checkout/update_checkout.py

@@ -166,7 +166,7 @@ def update_single_repository(pool_args):
                         result = shell.run(['git', 'rev-parse', checkout_target])
                         revision = result[0].strip()
                         shell.run(['git', 'checkout', revision], echo=True)
-                    except Exception as e:
+                    except Exception:
                         raise originalException
 
             # It's important that we checkout, fetch, and rebase, in order.