Raise minimum Python version

This was accidentally lowered in 5d3240d492 and 3.8 has gone EOL since then.

xb.ps1

@@ -14,7 +14,7 @@ foreach ($pythonExecutable in $pythonExecutables) {
     }
 }
 # Neither found, error and exit
-$pythonMinimumVer = 3,7
+$pythonMinimumVer = 3,9
 if (!$pythonPath) {
     Write-FatalError "ERROR: Python $($pythonMinimumVer[0]).$($pythonMinimumVer[1])+ must be installed and on PATH:`nhttps://www.python.org/"
 }