RAFlasher: add SCI (UART) method

[renesas-ra-flasher] / .flake8

[flake8]
max-line-length = 120
extend-ignore = E203, W503
exclude = .git,__pycache__,.venv
ignore = E261, E302, E305, E501, W291, W293, W503, W504, F401, F403, F841
inline-quotes = '
select = C,E,F,W,B,B950
per-file-ignores =
    # Example per-file ignores
    # __init__.py: F401