Ruby 3.0 support

[clarkedb]

Does the existing version (1.5.0) of postgres-copy work on Ruby 3.0? I didn't see anything in the docs confirming that it does. Also the CI only tests for ruby 2.7.

I would be happy to submit a PR to help if it needs work.

[diogob]

Some tests seem to be failing in my local machine under 3.0. I did not investigate it further. If you are willing to put the time I'd be glad to help you merge the code.

[clarkedb]

Some tests seem to be failing in my local machine under 3.0. I did not investigate it further. If you are willing to put the time I'd be glad to help you merge the code.

I've submitted a PR (#64) which makes the necessary changes and also modifies the CI workflow to run against multiple ruby versions.

[diogob]

@clarkedb I have released version 1.6.0 with your changes merged