style(tests): format import statements for better readability

from __future__ import annotations

import time

from concurrent.futures import (ProcessPoolExecutor, ThreadPoolExecutor,

                                as_completed)

from concurrent.futures import ProcessPoolExecutor, ThreadPoolExecutor, as_completed

from functools import partial

try:

import pytest

from pool_executors.pool_executors import (ExecutorType, SerialPoolExecutor,

                                           create_executor)

from pool_executors.pool_executors import ExecutorType, SerialPoolExecutor, create_executor

class TestSerialFuture: