Add an Audio display class

This is a first attempt at addressing issue 4241.
Fix unnecerary dependence to numpy
Move imports into methods and fix docstring typo
Fix talk about image embeding in the Audio class.

Too much copy paste I guess :)
Change to encodestring to b64encode to avoid newlines in the data URI

The point of this is to improve compatibility with older browsers.
also, encodestring is depricated in python 3.
Fix extra ) syntax error
Fix broken normalizing when signal array already has integer type

A case of unwanted integer division.
Speed up normalization if numpy is avaliable
Make it explicit that we don't support encoding stereo PCM signals
Remove support for encoding audio without numpy for performance
Add Audio display to the Rich display example notebook
Remove requirement for numpy module
Make examples in docstring  produce a less annoying sound.
More concise test summary info
Restore mention of number of test groups after success
Capture output from subprocs during test, and display on failure

This probably needs some more thought on synchronisation so we don't miss
critical bits of output as the test ends.
Don't try to wait for StreamCapturer that was never started
Don't show subprocess output if there isn't any
Use stream capturer for kernel tests
Allow checking for backports via milestone