feat: widen retry predicate to include ServiceUnavailable (#1641)

Expands retry.  It's possible in the normal lifecycle of an API frontend
for the intermediate response to indicate the API service is not ready.

related: internal issue 294103068

google/cloud/bigquery/retry.py

@@ -27,6 +27,7 @@
     exceptions.TooManyRequests,
     exceptions.InternalServerError,
     exceptions.BadGateway,
+    exceptions.ServiceUnavailable,
     requests.exceptions.ChunkedEncodingError,
     requests.exceptions.ConnectionError,
     requests.exceptions.Timeout,

tests/unit/test_retry.py

@@ -79,6 +79,12 @@ def test_w_unstructured_too_many_requests(self):
         exc = TooManyRequests("testing")
         self.assertTrue(self._call_fut(exc))
 
+    def test_w_unstructured_service_unavailable(self):
+        from google.api_core.exceptions import ServiceUnavailable
+
+        exc = ServiceUnavailable("testing")
+        self.assertTrue(self._call_fut(exc))
+
     def test_w_internalError(self):
         exc = mock.Mock(errors=[{"reason": "internalError"}], spec=["errors"])
         self.assertTrue(self._call_fut(exc))