Open file function when --onefile is used

When making a .EXE file with pyinstaller all files must be opened from the sys._MEIPASS "folder" and not the "./" one. To make it simpler it's better to have a openfile function like the following one:

filename = 'filename.ext'

        try:
            base_path = sys._MEIPASS
        except AttributeError:
            base_path = './'

        filename = os.path.join(base_path, filename)