fix(utils): handles Retry-After header as str value.

[anthonyalmarza]

We have an issue currently in our staging environment where the sdk is obfuscating the API rate limiting issue that we've run into.

The following is the error we're seeing.

TypeError: '>' not supported between instances of 'str' and 'int'
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "swap/transactions/views.py", line 125, in checkout
    obj.initialize_checkout(self.request.retailer)
  File "swap/transactions/models.py", line 623, in initialize_checkout
    if not self.get_shipping_address_for_participant(retailer).address_is_valid():
  File "swap/contacts/models.py", line 217, in address_is_valid
    return ShipmentApi.validate_shipping_address(self, save=save)
  File "swap/shipments/shipengine.py", line 110, in validate_shipping_address
    if return_value := self.validate_formatted_address(formatted_address):
  File "swap/shipments/shipengine.py", line 103, in validate_formatted_address
    return self.shipengine.validate_addresses([formatted_address])
  File "shipengine/shipengine.py", line 142, in validate_addresses
    return self.client.post(
  File "shipengine/http_client/client.py", line 61, in post
    return self._request_loop(
  File "shipengine/http_client/client.py", line 104, in _request_loop
    raise err
  File "shipengine/http_client/client.py", line 87, in _request_loop
    api_response = self._send_request(
  File "shipengine/http_client/client.py", line 145, in _send_request
    check_response_for_errors(
  File "shipengine/util/sdk_assertions.py", line 204, in check_response_for_errors
    if retry_after > config.timeout: