allow riscv64 package wheel

[ffgan]

PyPI recently added support for RISC-V, so we might consider adding support for RISC-V64.

Below are the CI results:
PyYAML CI

The results are generally well presented.

Other Info
Co-authored by: nijincheng@iscas.ac.cn;

[ffgan]

hi @nitzmahone, Could you please help to check this PR? If there is anything that needs to be modified, please feel free to contact me.

[nitzmahone]

At a glance this looks OK- I've assumed the requests for riscv64 wheels would be incoming for awhile now...

This will almost certainly be a 7.0 thing, as that's also where I'm currently planning to cut everything over to abi3/abi3t, which should significantly cut down the build and test overhead for adding new arches/platforms/Python versions.

[ffgan]

hi @nitzmahone. Hope you're doing well.
Is there anything that might be helpful on my end? Also, would it be possible to consider releasing an rv64 package for the 6.x version? There are quite a few people in the community who are really looking forward to it, and having access to it sooner would be wonderful for them.
The 7.x release seems like it might still be a little while away, so a 6.x rv64 package in the meantime could really benefit those who are eager to start using it.
Thank you so much for considering this! Please let me know if there's any way to support the effort.