Add 'virt' and 'group' settings for arm64-graviton2
https://github.com/pandas-dev/pandas/pull/40868 tried to change from `arm64` to `arm64-graviton2` but actually changed it to `amd64` because of the missing `virt` and `group` settings, as explained at https://blog.travis-ci.com/2020-09-11-arm-on-aws, section "Quick tips"
> Generally LGTM, I can't believe I missed that. However, Travis CI isn't running again(lack of credits again).

:_< thanks @martin-g  Welcome!