core: cleanup/sort imports according to ruff check --select I

my/core/tests/common.py

@@ -1,10 +1,9 @@
-from contextlib import contextmanager
 import os
+from contextlib import contextmanager
 from typing import Iterator, Optional
 
 import pytest
 
-
 V = 'HPI_TESTS_USES_OPTIONAL_DEPS'
 
 # TODO use it for serialize tests that are using simplejson/orjson?