CI: Re-enable Circle CI
I contacted CircleCI to check if they could be interested in sponsoring our ARM builds. Will update if I hear from them. For now looks like we're getting the 2,500 credits that @lithomas1 mentioned (not the 400,000 for open source projects for Linux builds). So, not much we can do with those.

I updated this PR so we don't add the CircleCI build, but still, it has everything else so the build passes when it can be added wherever. Only thing is that we now need to disable Circle CI, so the build doesn't fail after we remove the configuration. But only an admin of the pandas-dev organization can do it. @jreback do you mind going to https://app.circleci.com/settings/project/github/pandas-dev/pandas?return-to=https%3A%2F%2Fapp.circleci.com%2Fpipelines%2Fgithub%2Fpandas-dev%2Fpandas%2F30%2Fworkflows%2Ff9a6db17-510a-4eaf-a10c-b3a800e0a6c4%2Fjobs%2F30872 and clickig on "Stop builds" please?

_Originally posted by @datapythonista in https://github.com/pandas-dev/pandas/issues/41739#issuecomment-852446796_

We should re-enable Circle CI if we receive the Circle CI credits. 
cc @datapythonista for updates on this