Improved patches for sndhdr and imghdr by Victor Stinner, who writes:

- fix "h[sbseek] == b'\1'" and "ratecode = ord(h[sbseek+4])" in test_voc()
 - avoid division by zero
 - use startswith method: replace h[:2] == b'BM' by h.startswith(b'BM')
 - use aifc.open() instead of old aifc.openfp()
 - use ord(b'P') instead of ord('P')