Add Postal support

Thanks to @tiltec for researching, implementing, testing and documenting it.

tox.ini

@@ -16,7 +16,7 @@ envlist =
     # ... then prereleases (if available):
     djangoDev-py{38,39,310,py3}-all
     # ... then partial installation (limit extras):
-    django31-py37-{none,amazon_ses}
+    django31-py37-{none,amazon_ses,postal}
     # ... then older versions of some dependencies:
     django22-py37-all-old_urllib3
 
@@ -34,10 +34,12 @@ deps =
     mock
 extras =
     all,amazon_ses: amazon_ses
+    all,postal: cryptography
 setenv =
     # tell runtests.py to limit some test tags based on extras factor
-    none: ANYMAIL_SKIP_TESTS=amazon_ses
+    none: ANYMAIL_SKIP_TESTS=amazon_ses,postal
     amazon_ses: ANYMAIL_ONLY_TEST=amazon_ses
+    postal: ANYMAIL_ONLY_TEST=postal
 ignore_outcome =
     # CI that wants to handle errors itself can set TOX_FORCE_IGNORE_OUTCOME=false
     djangoDev: {env:TOX_FORCE_IGNORE_OUTCOME:true}