fix: add ConnectionError to default retry (#445)

This adds python built-in [ConnectionError](https://docs.python.org/3/library/exceptions.html#ConnectionError) to default retryable types. 
ConnectionError was recently added in the [BigQuery library](googleapis/python-bigquery#571) to allow retries. 

Fixes #426 🦕

google/cloud/storage/retry.py

@@ -21,7 +21,14 @@
 import json
 
 
-_RETRYABLE_TYPES = (
+# ConnectionError is a built-in exception only in Python3 and not in Python2.
+try:
+    _RETRYABLE_STDLIB_TYPES = (ConnectionError,)
+except NameError:
+    _RETRYABLE_STDLIB_TYPES = ()
+
+
+_RETRYABLE_TYPES = _RETRYABLE_STDLIB_TYPES + (
     api_exceptions.TooManyRequests,  # 429
     api_exceptions.InternalServerError,  # 500
     api_exceptions.BadGateway,  # 502
@@ -30,6 +37,7 @@
     requests.ConnectionError,
 )
 
+
 # Some retriable errors don't have their own custom exception in api_core.
 _ADDITIONAL_RETRYABLE_STATUS_CODES = (408,)
 

tests/unit/test_retry.py

@@ -19,6 +19,14 @@
 import mock
 
 
+try:
+    ConnectionError
+except NameError:
+    _HAS_STDLIB_CONNECTION_ERROR = False
+else:
+    _HAS_STDLIB_CONNECTION_ERROR = True
+
+
 class Test_should_retry(unittest.TestCase):
     def _call_fut(self, exc):
         from google.cloud.storage import retry
@@ -56,9 +64,22 @@ def test_w_google_api_call_error_miss(self):
         self.assertFalse(self._call_fut(exc))
 
     def test_w_requests_connection_error(self):
+        import requests
+
+        exc = requests.ConnectionError()
+        self.assertTrue(self._call_fut(exc))
+
+    def test_miss_w_stdlib_error(self):
         exc = ValueError("testing")
         self.assertFalse(self._call_fut(exc))
 
+    @unittest.skipUnless(
+        _HAS_STDLIB_CONNECTION_ERROR, "No builtin 'ConnectionError' in Python 2",
+    )
+    def test_w_stdlib_connection_error(self):
+        exc = ConnectionError()
+        self.assertTrue(self._call_fut(exc))
+
 
 class TestConditionalRetryPolicy(unittest.TestCase):
     def _make_one(self, retry_policy, conditional_predicate, required_kwargs):