MAINT: io: Give an informative error when attempting to read a 24 bit wav file.

Also add a note to the docstring of wavfile.read that 24 bit files are not
supported.
MAINT: io: PEP8 clean up in wavfile.py
Merge pull request #4266 from WarrenWeckesser/wav-24bit-error

 MAINT: io: Give an informative error when attempting to read a 24 bit wav file.