Archive this repository

[lukesneeringer]

@jonparrott Since we have done a release-the-world, I think no current versions should depend on gax anymore. GitHub now has an "archive" feature that will mark this repo read-only. Should I update the README to point to api_core and then do that?

[theacodes]

It seems some of the manual clients in google-cloud-python are still using gax.

[lukesneeringer]

Ack.