Retry email sends on a delay if mailgun is having uptime issues

[sentry-io]

Sentry Issue: DISCUSSIONS-2N5

WantReadError: 
  File "urllib3/contrib/pyopenssl.py", line 456, in wrap_socket
    cnx.do_handshake()
  File "OpenSSL/SSL.py", line 1806, in do_handshake
    self._raise_ssl_error(self._ssl, result)
  File "OpenSSL/SSL.py", line 1521, in _raise_ssl_error
    raise WantReadError()

timeout: select timed out
(1 additional frame(s) were not displayed)
...
  File "urllib3/connectionpool.py", line 839, in _validate_conn
    conn.connect()
  File "newrelic/hooks/external_httplib.py", line 33, in httplib_connect_wrapper
    return wrapped(*args, **kwargs)
  File "urllib3/connection.py", line 344, in connect
    ssl_context=context)
  File "urllib3/util/ssl_.py", line 347, in ssl_wrap_socket
    return context.wrap_socket(sock, server_hostname=server_hostname)
  File "urllib3/contrib/pyopenssl.py", line 459, in wrap_socket
    raise timeout('select timed out')

ReadTimeoutError: HTTPSConnectionPool(host='api.mailgun.net', port=443): Read timed out. (read timeout=30)
(4 additional frame(s) were not displayed)
...
  File "urllib3/connectionpool.py", line 600, in urlopen
    chunked=chunked)
  File "newrelic/hooks/external_urllib3.py", line 21, in _nr_wrapper_make_request_
    return wrapped(*args, **kwargs)
  File "newrelic/hooks/external_urllib3.py", line 21, in _nr_wrapper_make_request_
    return wrapped(*args, **kwargs)
  File "urllib3/connectionpool.py", line 346, in _make_request
    self._raise_timeout(err=e, url=url, timeout_value=conn.timeout)
  File "urllib3/connectionpool.py", line 306, in _raise_timeout
    raise ReadTimeoutError(self, url, "Read timed out. (read timeout=%s)" % timeout_value)

ReadTimeout: HTTPSConnectionPool(host='api.mailgun.net', port=443): Read timed out. (read timeout=30)
(2 additional frame(s) were not displayed)
...
  File "newrelic/api/external_trace.py", line 62, in dynamic_wrapper
    return return_value(trace, lambda: wrapped(*args, **kwargs))
  File "newrelic/api/coroutine_trace.py", line 171, in return_value
    return fn()
  File "newrelic/api/external_trace.py", line 62, in <lambda>
    return return_value(trace, lambda: wrapped(*args, **kwargs))
  File "requests/sessions.py", line 646, in send
    r = adapter.send(request, **kwargs)
  File "requests/adapters.py", line 529, in send
    raise ReadTimeout(e, request=request)

AnymailRequestsAPIError: Error posting to https://api.mailgun.net/v3/mail.open.mit.edu/messages:
requests.exceptions.ReadTimeout: HTTPSConnectionPool(host='api.mailgun.net', port=443): Read timed out. (read timeout=30)
Sending a message to User <[email protected]> from MIT Open <[email protected]
(4 additional frame(s) were not displayed)
...
  File "django/core/mail/message.py", line 291, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "anymail/backends/base.py", line 93, in send_messages
    sent = self._send(message)
  File "anymail/backends/base_requests.py", line 55, in _send
    return super(AnymailRequestsBackend, self)._send(message)
  File "anymail/backends/base.py", line 123, in _send
    response = self.post_to_esp(payload, message)
  File "anymail/backends/base_requests.py", line 76, in post_to_esp
    raised_from=err, email_message=message, payload=payload)

Error sending email '[MIT Open] Top Stories for You' to ["User <[email protected]>"]