Fix linters for helpers.py and ros2node/api/__init__.py

[Crola1702]

Linux debug CI shows test regressions with test_flake8:

Reference build: Linux Debug #2589

ros2cli.ros2cli.test.test_flake8.test_flake8

test/test_flake8.py:23: in test_flake8
    assert rc == 0, \
E   AssertionError: Found 1 code style errors / warnings:
E     ./ros2cli/helpers.py:46:1: E302 expected 2 blank lines, found 1
E   assert 1 == 0

ros2node.ros2node.test.test_flake8.test_flake8

test/test_flake8.py:23: in test_flake8
    assert rc == 0, \
E   AssertionError: Found 2 code style errors / warnings:
E     ./ros2node/api/__init__.py:15:1: F401 'time' imported but unused
E     ./ros2node/api/__init__.py:17:1: I100 Import statements are in the wrong order. 'from collections import namedtuple' should be before 'import time'
E   assert 1 == 0

[mjcarroll]

Duplicate of #807 (or vice-versa, they were opened at the same time!)

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows

[clalancette]

The failing tests on Linux are a separate problem. This looks like it fixes the flake8 issues, so going ahead and merging it. Thanks @Crola1702 !