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

tox.ini

@@ -1,17 +1,20 @@
 [tox]
 envlist =
+    # Factors: django-python-extras
     # Test these environments first, to catch most errors early...
     lint
-    django21-py36
-    django111-py27
+    django21-py36-all
+    django111-py27-all
     docs
     # ... then test all the other supported combinations:
-    django21-py{35,37,py3}
-    django20-py{35,36,py3}
-    django111-py{34,35,36,py}
+    django21-py{35,37,py3}-all
+    django20-py{35,36,py3}-all
+    django111-py{34,35,36,py}-all
     # ... then prereleases (if available):
-    django22-py{35,36,37,py3}
-    djangoMaster-py{36,37}
+    django22-py{35,36,37,py3}-all
+    djangoMaster-py{36,37}-all
+    # ... then partial installation (limit extras):
+    django21-py37-{none,amazon_ses,sparkpost}
 
 [testenv]
 deps =
@@ -20,19 +23,27 @@ deps =
     django21: django~=2.1.0
     django22: django>=2.2a1
     djangoMaster: https://github.com/django/django/tarball/master
-    # testing dependencies (duplicates setup.py tests_require):
+    # testing dependencies (duplicates setup.py tests_require, less optional extras):
     mock
-    boto3
-    sparkpost
+extras =
+    all,amazon_ses: amazon_ses
+    all,sparkpost: sparkpost
+setenv =
+    # tell runtests.py to limit some test tags based on extras factor
+    none: ANYMAIL_SKIP_TESTS=amazon_ses,sparkpost
+    amazon_ses: ANYMAIL_ONLY_TEST=amazon_ses
+    sparkpost: ANYMAIL_ONLY_TEST=sparkpost
 ignore_outcome =
-    djangoMaster: True
-usedevelop = True
-args_are_paths = False
+    # CI that wants to handle errors itself can set TOX_FORCE_IGNORE_OUTCOME=false
+    djangoMaster: {env:TOX_FORCE_IGNORE_OUTCOME:true}
+args_are_paths = false
+commands_pre =
+    python -VV
 commands =
-    python --version
-    # pip install .[mailgun,...,sparkpost]  ## usedevelop=True + manual deps is much faster on repeat runs
     python runtests.py {posargs}
 passenv =
+    ANYMAIL_ONLY_TEST
+    ANYMAIL_SKIP_TESTS
     RUN_LIVE_TESTS
     CONTINUOUS_INTEGRATION
     AMAZON_SES_TEST_*
@@ -46,7 +57,7 @@ passenv =
 
 [testenv:lint]
 basepython = python3
-skip_install = True
+skip_install = true
 passenv =
     CONTINUOUS_INTEGRATION
 	# (but not any of the live test API keys)
@@ -59,7 +70,7 @@ commands =
 
 [testenv:docs]
 basepython = python3
-skip_install = True
+skip_install = true
 passenv =
     CONTINUOUS_INTEGRATION
 	# (but not any of the live test API keys)
@@ -78,20 +89,3 @@ commands =
     /bin/bash -c 'python setup.py --long-description \
                   | rst2html5.py --config=docs/_readme/docutils.cfg \
                   > {env:DOCS_BUILD_DIR}/readme.html'
-
-[travis]
-unignore_outcomes = True
-python =
-    3.6: py36, lint, docs
-
-[travis:env]
-DJANGO =
-    1.11: django111
-    2.0: django20
-    2.1: django21
-    2.2: django22
-    master: djangoMaster
-LINT_AND_DOCS =
-    true: lint, docs
-    docs: docs
-    lint: lint