Add static method `from_bytes` to `SoundSDL2` to initialize an instance from audio sample in memory

[sassanh]

Kivy allows loading audio samples from filesystem by filename using the source property of Sound object, but sometimes the audio samples are generated in the memory, for example it happens when using text to speech engines like Picovoice's Orca or Piper and in environments like RPi running on a SD-card, writing the samples from memory to filesystem in a temporary file and then asking Kivy to instruct SDL to read that file again into memory may cause some unwanted side effects like delay or exhaustion of the SD-card.

from_bytes static method added in this pull request, lets the user instantiate SoundSDL2 with data coming directly from memory.

I considered re-using the source property by adding a "mem:" prefix or something like that as a signal for load method, but it's a StringProperty and only accepts str, not bytes.

Maintainer merge checklist

• [ ] Title is descriptive/clear for inclusion in release notes.
• [ ] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [ ] Added to the milestone version it was merged into.
• [ ] Unittests are included in PR.
• [ ] Properly documented, including versionadded, versionchanged as needed.