You need to have win32com installed on your machine. Here is my code:
import win32com.client as win32
fname = "full+path+to+xls_file"
excel = win32.gencache.EnsureDispatch('Excel.Application')
wb = excel.Workbooks.Open(fname)
wb.SaveAs(fname+"x", FileFormat = 51) #FileFormat = 51 is for .xlsx extension
wb.Close() #FileFormat = 56 is for .xls extension
excel.Application.Quit()