Make json_tricks work with async/await

[mverleg]

Not sure this is worthwhile, because GIL prevents any separate thread for performing well. And it's not safe to share mutable data anyway.

[mverleg]

I think when I originally created this, it was about offloading the encoding/decoding itself to another thread or process, and making an async interface.

I'm not sure that's worthwhile, for the reasons mentioned.

However, it would be easy to create an async wrapper for the file reading and writing, on Python versions that support it. That might be useful.