Add timeout to all Requests calls

Use a default timeout of 30 seconds for all requests, and add a
REQUESTS_TIMEOUT Anymail setting to override.

(I'm making a judgement call that this is not a breaking change in the
real world, and not bumping the major version. Theoretically, it could
affect you if your network somehow takes >30s to connect to your ESP,
but eventually succeeds. If so, set REQUESTS_TIMEOUT to None to restore
the earlier behavior.)

Fixes #80.

tests/test_base_backends.py

@@ -0,0 +1,65 @@
+from django.test import override_settings
+
+from anymail.backends.base_requests import AnymailRequestsBackend, RequestsPayload
+from anymail.message import AnymailMessage, AnymailRecipientStatus
+
+from .mock_requests_backend import RequestsBackendMockAPITestCase
+
+
+class MinimalRequestsBackend(AnymailRequestsBackend):
+    """(useful only for these tests)"""
+
+    esp_name = "Example"
+
+    def __init__(self, **kwargs):
+        super(MinimalRequestsBackend, self).__init__("https://esp.example.com/api/", **kwargs)
+
+    def build_message_payload(self, message, defaults):
+        return MinimalRequestsPayload(message, defaults, self)
+
+    def parse_recipient_status(self, response, payload, message):
+        return {'to@example.com': AnymailRecipientStatus('message-id', 'sent')}
+
+
+class MinimalRequestsPayload(RequestsPayload):
+    def init_payload(self):
+        pass
+
+    def _noop(self, *args, **kwargs):
+        pass
+
+    set_from_email = _noop
+    set_recipients = _noop
+    set_subject = _noop
+    set_reply_to = _noop
+    set_extra_headers = _noop
+    set_text_body = _noop
+    set_html_body = _noop
+    add_attachment = _noop
+
+
+@override_settings(EMAIL_BACKEND='tests.test_base_backends.MinimalRequestsBackend')
+class RequestsBackendBaseTestCase(RequestsBackendMockAPITestCase):
+    """Test common functionality in AnymailRequestsBackend"""
+
+    def setUp(self):
+        super(RequestsBackendBaseTestCase, self).setUp()
+        self.message = AnymailMessage('Subject', 'Text Body', 'from@example.com', ['to@example.com'])
+
+    def test_minimal_requests_backend(self):
+        """Make sure the testing backend defined above actually works"""
+        self.message.send()
+        self.assert_esp_called("https://esp.example.com/api/")
+
+    def test_timeout_default(self):
+        """All requests have a 30 second default timeout"""
+        self.message.send()
+        timeout = self.get_api_call_arg('timeout')
+        self.assertEqual(timeout, 30)
+
+    @override_settings(ANYMAIL_REQUESTS_TIMEOUT=5)
+    def test_timeout_setting(self):
+        """You can use the Anymail setting REQUESTS_TIMEOUT to override the default"""
+        self.message.send()
+        timeout = self.get_api_call_arg('timeout')
+        self.assertEqual(timeout, 5)