get_png_dim(filepath:str) -> (int)
    Finds the dimension of a PNG.
    :param filepath: file path of the PNG.
    :returns: (width, height).

def get_png_dim(filepath: str) -> int:
    """
    get_png_dim(filepath:str) -> (int)
    Finds the dimension of a PNG.
    :param filepath: file path of the PNG.
    :returns: (width, height).
    """
    if not imghdr.what(filepath) == 'png':
        raise Exception("not PNG")
    head = open(filepath, 'rb').read(24)
    return struct.unpack('!II', head[16:24])