Issue #18950: Fix miscellaneous bugs in the sunau module.

Au_read.readframes() now updates current file position and reads correct
number of frames from multichannel stream.  Au_write.writeframesraw() now
correctly updates current file position.  Au_read.getnframes() now returns an
integer (as in Python 2).  Au_read and Au_write now correctly works with file
object if start file position is not a zero.

diff --cc Lib/sunau.py
Simple merge

diff --cc Misc/NEWS
Simple merge