Include GraalPy as an "Alternative Python Implementation"

[timfel]

Summary

GraalPy is a Python 3 implementation written in Java, taking advantage of the Graal JIT compiler and the Truffle language implementation framework.

This is a request to include it in the Alternative Python Implementations page.

Disclaimer: I work on GraalPy at Oracle.

[hugovk]

Thanks, I've added it.

I also removed Stackless Python because the repo has been archived:

Development of this project has now stopped, and the project is archived."

https://github.com/stackless-dev/stackless