Port brigadier to Python 3 and fix Windows download flow

[illuminatus503]

This ports the main brigadier script to Python 3 while preserving compatibility with older plist and urllib APIs where possible.

On Windows, it also hardens the download/extract flow:

• use requests for Python 3 downloads and keep progress reporting working
• provision 7-Zip from the MSI when needed, with a fallback to an administrative extract if 7z.exe is not made available after install
• allow reuse of an existing output directory when Windows keeps handles open instead of aborting the run
• mirror the 7-Zip and landing-directory handling improvements in brigadier.ps1

Validation:

• .venv-py38\\Scripts\\python.exe -m py_compile brigadier
• .venv-py38\\Scripts\\python.exe brigadier --version
• PowerShell parser check for brigadier.ps1

I did not run an end-to-end Boot Camp download/install in this environment.