Initialize PyAudio in Mic.__init__

With this PR, we'll initialize PyAudio only once in the `__init__()` method of the `Mic` class. PyAudio's `terminate()` method will be called in the `__del__()` method of the `Mic` class accordingly.

This way, we'll improve Performance and also stop printing ALSA/JACK debug messages all over the place (PyAudio does this on every initialization).