fix flak8 flagged errors

the auto build process (3.8) in GitHub reported
Run flake8 --ignore=E501,E126,E127 arrowhead_client/ examples/
flake8 --ignore=E501,E1[2](https://github.com/arrowhead-f/client-library-python/actions/runs/8906906566/job/24518758592#step:5:2)6,E127 arrowhead_client/ examples/
shell: /usr/bin/bash -e {0}
env:
pythonLocation: /opt/hostedtoolcache/Python/[3](https://github.com/arrowhead-f/client-library-python/actions/runs/8906906566/job/24518758592#step:5:3).8.18/x64
LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.8.18/x6[4](https://github.com/arrowhead-f/client-library-python/actions/runs/8906906566/job/24518758592#step:5:4)/lib
arrowhead_client/client/client_async.py:8:1: F401 'arrowhead_client.provider.implementations.fastapi_provider.FastapiProvider' imported but unused
arrowhead_client/client/client_core.py:4:1: F401 'typing.Optional' imported but unused
arrowhead_client/client/client_sync.py:1:1: F401 'typing.Optional' imported but unused
arrowhead_client/client/client_sync.py:2:1: F401 'warnings' imported but unused
arrowhead_client/client/client_sync.py:12:1: E302 expected 2 blank lines, found 1
arrowhead_client/constants.py:[5](https://github.com/arrowhead-f/client-library-python/actions/runs/8906906566/job/24518758592#step:5:5)2:1: E302 expected 2 blank lines, found 1
Error: Process completed with exit code 1.

GitHub Actions の失敗に示されているとおり、`flake8 --ignore=E501,E126,E127 arrowhead_client/ examples/` を実行します。`arrowhead_client/client/client_async.py`、`client_core.py`、`client_sync.py`、`arrowhead_client/constants.py` にある報告された import と空白を確認します。コマンドが一覧にある F401 および E302 エラーなしで完了すれば完了です。

索引モデルが issue の本文から書いたものです。