Ruby 3 support

[fpietka]

Hi,

I was wondering if you planned on supporting Ruby 3.0. I see that net-http-persistent support Ruby 3.0 starting latest 4.0.1:
https://github.com/drbrain/net-http-persistent/blob/master/History.txt#L1-L5

Not sure if there are other things to check along the migration.