deps: allow 2.x versions of google-api-core, google-cloud-core, `…

…google-resumable-media` (googleapis#770)

**Note**: PR is empty because this is purely to make sure CHANGELOG is updated. Follow-up to googleapis#768

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- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/python-bigquery/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
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Fixes googleapis#767  🦕