bpo-32718: Make Activate.ps1 for venv cross-platform and available on all platforms (GH-9321)

PowerShell Core 6.1 is the cross-platform port of Windows PowerShell. This change updates Activate.ps1 to not make Windows assumptions as well as installing it into the bin/Scripts directory on all operating systems.

Requires PowerShell Core 6.1 for proper readline support once the shell has been activated for the virtual environment.