nvwave.playWaveFile: async > asynchronous (#8647)
* nvWave.playWaveFile: async > asynchronous. Re #8607.
Python 3.5 introduces 'async' and 'await' keywords to deal with asynchronous generators and other possibiliites. Since Python 3.7, use of these keywords as variable names is no longer allowed. In NVDA code, nvWave.playWaveFile is affected, so rename 'async' to 'asynchronous'.
* nvwave.playWaveFile: document 'asynchronous' keyword.
Reviewed by Leonard de Ruijter (Babbage): document the renamed keyword arg.
* Speech commands: async -> asynchronous.
* nvwave.playWaveFile: correct argument name in docstring.
