Smtptester

- Require Python `^3.8`

- Use QT6 (PySide6)
- Use [Poetry](https://python-poetry.org/)
- Added short options:
- `-l`: Log level
- `-s`: Sender
- `-m`: Message
- `-d`: DNS host
- `-h`: SMTP host
- `-u`: SMTP auth user
- `-p`: SMTP auth password
- Removed colored logs in GUI
- Added type annotations

- Fix for pip not installing dependencies

- Complete rewrite
- Support Python 3.7+
- Add unit tests
- Add CLI interface
- Switch from wxWidgets to QT5 (PySide2)

- Fix incorrect name in title bar

- Fix missing icon on some platforms