Update README.md to correct links and remove Nebula Pro section

Bump version: 2.0.0b18 -> 2.0.0b19

Merge pull request #104 from berylliumsec/supporting_older_python_versions

Update release notes to highlight support for older Python versions

Update Python compatibility and dependencies for older versions

Bump version: 2.0.0b17 -> 2.0.0b18

Merge pull request #102 from berylliumsec/house-keeping

Update README to specify Python version range and correct environment variable name for OpenAI API key

Update release notes to reflect recent enhancements and fixes, including terminal emulator rebuild, engagement dialog improvements, and window lifecycle management.

Refactor main window loading logic to improve progress window handling

Add user settings to remember last engagement directory

Initialize help_window attribute in Nebula class

Refactor clear_screen method to clear central display area in both MainWindow and TerminalEmulator

Refine Python version constraint and enhance settings dialog functionality

Update dependencies and improve terminal backend handling