CI: Refactor workflows, update test matrix

* Split test, integration-test, and release workflows,
  simplifying config and logic for each.
* Use environment to separate secrets for release workflow.
* Update test matrix with newer Django and Python versions.

tox.ini

@@ -3,11 +3,12 @@ envlist =
     # Factors: django-python-extras
     # Test these environments first, to catch most errors early...
     lint
-    django32-py39-all
+    django40-py310-all
     django20-py35-all
     docs
     # ... then test all the other supported combinations:
-    django32-py{36,37,38,py3}-all
+    django40-py{38,39,py3}-all
+    django32-py{36,37,38,39,py3}-all
     django31-py{36,37,38,39,py3}-all
     django30-py{36,37,38,39,py3}-all
     django22-py{35,36,37,38,39,py3}-all
@@ -16,7 +17,7 @@ envlist =
     # ... then prereleases (if available):
     djangoDev-py{38,39,310,py3}-all
     # ... then partial installation (limit extras):
-    django31-py37-{none,amazon_ses,postal}
+    django40-py310-{none,amazon_ses,postal}
     # ... then older versions of some dependencies:
     django22-py37-all-old_urllib3
 
@@ -28,6 +29,7 @@ deps =
     django30: django~=3.0.0
     django31: django~=3.1.0
     django32: django~=3.2.0
+    django40: django~=4.0.0
     djangoDev: https://github.com/django/django/tarball/main
     old_urllib3: urllib3<1.25
 extras =
@@ -40,7 +42,14 @@ setenv =
     # tell runtests.py to limit some test tags based on extras factor
     none: ANYMAIL_SKIP_TESTS=amazon_ses,postal
     amazon_ses: ANYMAIL_ONLY_TEST=amazon_ses
+    mailgun: ANYMAIL_ONLY_TEST=mailgun
+    mailjet: ANYMAIL_ONLY_TEST=mailjet
+    mandrill: ANYMAIL_ONLY_TEST=mandrill
     postal: ANYMAIL_ONLY_TEST=postal
+    postmark: ANYMAIL_ONLY_TEST=postmark
+    sendgrid: ANYMAIL_ONLY_TEST=sendgrid
+    sendinblue: ANYMAIL_ONLY_TEST=sendinblue
+    sparkpost: ANYMAIL_ONLY_TEST=sparkpost
 ignore_outcome =
     # CI that wants to handle errors itself can set TOX_FORCE_IGNORE_OUTCOME=false
     djangoDev: {env:TOX_FORCE_IGNORE_OUTCOME:true}