Invoking a PowerShell script from Python

I'm trying to start a PowerShell script from python like this:

psxmlgen = subprocess.Popen([r'C:\WINDOWS\system32\WindowsPowerShell\v1.0\powershell.exe',
                             './buildxml.ps1',
                             arg1, arg2, arg3], cwd=os.getcwd())
result = psxmlgen.wait()

The problem is that I get the following error:

File C:\Users\sztomi\workspace\myproject\buildxml.ps1 cannot be loaded because the execution of scripts is disabled on this system. Please see "get-help about_signing" for more details.

DESPITE the fact that I did enable running scripts in Powershell a long time ago by typing Set-ExecutionPolicy Unrestriced in an administrator-ran PS terminal (and did again, just to make sure). The powershell executable is the same that the shortcut in start menu points to. Get-ExecutionPolicy correctly reports Unrestricted no matter if I ran PowerShell as admin or not.

How can I execute a PS script correctly from Python?