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_sparkpost_backend.py

@@ -8,11 +8,9 @@ import os
 import requests
 import six
 from django.core import mail
-from django.test import SimpleTestCase
-from django.test.utils import override_settings
+from django.test import SimpleTestCase, override_settings, tag
 from django.utils.timezone import get_fixed_timezone, override as override_current_timezone, utc
 from mock import patch
-from sparkpost.exceptions import SparkPostAPIException
 
 from anymail.exceptions import (AnymailAPIError, AnymailUnsupportedFeature, AnymailRecipientsRefused,
                                 AnymailConfigurationError, AnymailInvalidAddress)
@@ -21,6 +19,7 @@ from anymail.message import attach_inline_image_file
 from .utils import AnymailTestMixin, decode_att, SAMPLE_IMAGE_FILENAME, sample_image_path, sample_image_content
 
 
+@tag('sparkpost')
 @override_settings(EMAIL_BACKEND='anymail.backends.sparkpost.EmailBackend',
                    ANYMAIL={'SPARKPOST_API_KEY': 'test_api_key'})
 class SparkPostBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
@@ -48,6 +47,7 @@ class SparkPostBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
         return self.mock_send.return_value
 
     def set_mock_failure(self, status_code=400, raw=b'{"errors":[{"message":"test error"}]}', encoding='utf-8'):
+        from sparkpost.exceptions import SparkPostAPIException
         # Need to build a real(-ish) requests.Response for SparkPostAPIException
         response = requests.Response()
         response.status_code = status_code
@@ -82,6 +82,7 @@ class SparkPostBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
             raise AssertionError(msg or "ESP API was called and shouldn't have been")
 
 
+@tag('sparkpost')
 class SparkPostBackendStandardEmailTests(SparkPostBackendMockAPITestCase):
     """Test backend support for Django standard email features"""
 
@@ -325,6 +326,7 @@ class SparkPostBackendStandardEmailTests(SparkPostBackendMockAPITestCase):
             self.message.send()
 
 
+@tag('sparkpost')
 class SparkPostBackendAnymailFeatureTests(SparkPostBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
@@ -545,6 +547,7 @@ class SparkPostBackendAnymailFeatureTests(SparkPostBackendMockAPITestCase):
     #   modify those errors.
 
 
+@tag('sparkpost')
 class SparkPostBackendRecipientsRefusedTests(SparkPostBackendMockAPITestCase):
     """Should raise AnymailRecipientsRefused when *all* recipients are rejected or invalid"""
 
@@ -586,6 +589,7 @@ class SparkPostBackendRecipientsRefusedTests(SparkPostBackendMockAPITestCase):
         self.assertEqual(sent, 1)  # refused message is included in sent count
 
 
+@tag('sparkpost')
 @override_settings(EMAIL_BACKEND="anymail.backends.sparkpost.EmailBackend")
 class SparkPostBackendConfigurationTests(SimpleTestCase, AnymailTestMixin):
     """Test various SparkPost client options"""