Adjust tag for api_core 1.7.0 release.
Adjust tag for api_core 1.7.0 release.

GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tseaver/google-cloud-python