Python/xenia-build/xb fixes

Use variable for Python version to make upgrading easier.
xb.bat:
 Update copyright date.
 Add candidate paths.
xb.ps1
 Properly use found python executable.
 More consistency with .bat.
 Don't spew unnecessary errors, etc.
 EOF newline.

xenia-build

@@ -134,8 +134,9 @@ def main():
         print('')
 
     # Check python version.
-    if not sys.version_info[:2] >= (3, 6):
-        print('ERROR: Python 3.6+ must be installed and on PATH')
+    python_minimum_ver=3,6
+    if not sys.version_info[:2] >= (python_minimum_ver[0], python_minimum_ver[1]):
+        print('ERROR: Python ', python_minimum_ver[0], '.', python_minimum_ver[1], '+ must be installed and on PATH', sep='')
         sys.exit(1)
 
     # Grab Visual Studio version and execute shell to set up environment.
@@ -1830,4 +1831,3 @@ class DevenvCommand(Command):
 
 if __name__ == '__main__':
     main()
-