Test without optional packages

Tox:
* Add Tox factor for extras (all, none, individual ESP).
  For now, only break out ESPs that have specific extra
  dependencies (amazon_ses, sparkpost).
* Install most package dependencies (including extras)
  through the package itself.
* Use new runtests.py environment vars to limit test tags
  when Tox isn't installing all extras.

Travis:
* Rework matrix to request specific TOXENVs directly;
  drop tox-travis.

Test runner (runtests.py):
* Centralize RUN_LIVE_TESTS logic in runtests.py
* Add ANYMAIL_ONLY_TEST and ANYMAIL_SKIP_TESTS env vars
  (comma-separated lists of tags)

Test implementations:
* Tag all ESP-specific tests with ESP
* Tag live tests with "live"
* Don't import ESP-specific packages at test module level. 
  (Test discovery imports test modules before tag-based filtering.)

Closes #104

tests/test_amazon_ses_backend.py

@@ -5,13 +5,10 @@ import json
 from datetime import datetime
 from email.mime.application import MIMEApplication
 
-import botocore.config
-import botocore.exceptions
 import six
 from django.core import mail
 from django.core.mail import BadHeaderError
-from django.test import SimpleTestCase
-from django.test.utils import override_settings
+from django.test import SimpleTestCase, override_settings, tag
 from mock import ANY, patch
 
 from anymail.exceptions import AnymailAPIError, AnymailUnsupportedFeature
@@ -20,6 +17,7 @@ from anymail.message import attach_inline_image_file, AnymailMessage
 from .utils import AnymailTestMixin, SAMPLE_IMAGE_FILENAME, sample_image_content, sample_image_path
 
 
+@tag('amazon_ses')
 @override_settings(EMAIL_BACKEND='anymail.backends.amazon_ses.EmailBackend')
 class AmazonSESBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
     """TestCase that uses the Amazon SES EmailBackend with a mocked boto3 client"""
@@ -61,8 +59,9 @@ class AmazonSESBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
         return mock_operation.return_value
 
     def set_mock_failure(self, response, operation_name="send_raw_email"):
+        from botocore.exceptions import ClientError
         mock_operation = getattr(self.mock_client_instance, operation_name)
-        mock_operation.side_effect = botocore.exceptions.ClientError(response, operation_name=operation_name)
+        mock_operation.side_effect = ClientError(response, operation_name=operation_name)
 
     def get_session_params(self):
         if self.mock_session.call_args is None:
@@ -111,6 +110,7 @@ class AmazonSESBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
             raise AssertionError(msg or "ESP API was called and shouldn't have been")
 
 
+@tag('amazon_ses')
 class AmazonSESBackendStandardEmailTests(AmazonSESBackendMockAPITestCase):
     """Test backend support for Django standard email features"""
 
@@ -318,6 +318,7 @@ class AmazonSESBackendStandardEmailTests(AmazonSESBackendMockAPITestCase):
                               headers={"X-Header": "custom header value\r\ninjected"}).send()
 
 
+@tag('amazon_ses')
 class AmazonSESBackendAnymailFeatureTests(AmazonSESBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
@@ -589,6 +590,7 @@ class AmazonSESBackendAnymailFeatureTests(AmazonSESBackendMockAPITestCase):
         self.assertEqual(self.message.anymail_status.esp_response, response_content)
 
 
+@tag('amazon_ses')
 class AmazonSESBackendConfigurationTests(AmazonSESBackendMockAPITestCase):
     """Test configuration options"""
 
@@ -635,7 +637,8 @@ class AmazonSESBackendConfigurationTests(AmazonSESBackendMockAPITestCase):
 
     def test_client_params_in_connection_init(self):
         """You can also supply credentials specifically for a particular EmailBackend connection instance"""
-        boto_config = botocore.config.Config(connect_timeout=30)
+        from botocore.config import Config
+        boto_config = Config(connect_timeout=30)
         conn = mail.get_connection(
             'anymail.backends.amazon_ses.EmailBackend',
             client_params={"aws_session_token": "test-session-token", "config": boto_config})