Pin 'google-api_core >= 1.7.0'. (#6937)
Pin 'google-api_core >= 1.7.0'. (#6937)

That release added the 'from_timestamp_pb' / 'to_timestamp_pb' methods to 'DatetimeWithNanos'.
tseaver/google-cloud-python,tseaver/google-cloud-python,googleapis/google-cloud-python,tseaver/google-cloud-python,tswast/google-cloud-python,tswast/google-cloud-python,tswast/google-cloud-python,GoogleCloudPlatform/gcloud-python,googleapis/google-cloud-python,GoogleCloudPlatform/gcloud-python