gbinder: fix methods that accept or return memory buffers

[NotKit]

Use memoryviews to represent opaque binary data buffers so they can be accessed from Python.

Note: This is an API-breaking change. However, it's safe to assume there should be no affected users since the previous implementation would crash due to incorrect casting of raw C memory to Python objects.

[aleasto]

I don't speak cpython very well. But I agree that it's not a problem to break API like this.

@erfanoabdi are you able to review this? Otherwise please feel free to merge yourself @NotKit :)

[JamiKettunen]

I suppose that implies upstream for the repo moves here along with future development (which maybe should be done)? this repo is missing https://github.com/erfanoabdi/gbinder-python/releases/tag/1.1.2 (the tag itself, not the commits) for example and has bullseye as the only branch which is perhaps strange. and of course the obvious: this is a fork of Erfan's repo (which is the ones distros package afaik) while that one is still not archived or anything :p

[aleasto]

Ugh I forgot that upstream is technically erfan's