macOS: NSSpeechSynthesizer (nsss) is deprecated by Apple

[cclauss]

It still works as expected today but should future-proofing work be done?
https://developer.apple.com/documentation/appkit/nsspeechsynthesizer

pyttsx3/tests/test_pyttsx3.py

Line 84 in 51e6fb8

def test_saving_to_file(engine, tmp_path):

Generates wave.Error: file does not start with RIFF id

[willwade]

#336