Teardown of `blobs_to_delete` fixture flakes with TimeoutError.

[tseaver]

From this Kokoro failure:

self = <urllib3.connectionpool.HTTPSConnectionPool object at 0x7fdd9aa453a0>
err = timeout('The read operation timed out')
url = '/storage/v1/b/gcp-systest-kms-1626816467428/o/test-blob?generation=1626816473471724&prettyPrint=false'
timeout_value = 60

    def _raise_timeout(self, err, url, timeout_value):
        """Is the error actually a timeout? Will raise a ReadTimeout or pass"""

        if isinstance(err, SocketTimeout):
>           raise ReadTimeoutError(
                self, url, "Read timed out. (read timeout=%s)" % timeout_value
            )
E           urllib3.exceptions.ReadTimeoutError: HTTPSConnectionPool(host='storage.googleapis.com', port=443): Read timed out. (read timeout=60)

.nox/system-3-8/lib/python3.8/site-packages/urllib3/connectionpool.py:336: ReadTimeoutError

During handling of the above exception, another exception occurred:

    @pytest.fixture(scope="function")
    def blobs_to_delete():
        blobs_to_delete = []

        yield blobs_to_delete

        for blob in blobs_to_delete:
>           _helpers.delete_blob(blob)