Tests: tox and mock cleanup

* Remove mock dependency
  (mock is now part of standard unittest package)
* Cleanup tox dependency installation logic

tox.ini

@@ -27,14 +27,15 @@ deps =
     django22: django~=2.2.0
     django30: django~=3.0.0
     django31: django~=3.1.0
-    django32: django>=3.2a1
+    django32: django~=3.2.0
     djangoDev: https://github.com/django/django/tarball/main
     old_urllib3: urllib3<1.25
-    # testing dependencies (duplicates setup.py tests_require, less optional extras):
-    mock
 extras =
+    # install [test] extras, unconditionally
+    test
+    # install [esp_name] extras only when testing "all" or esp_name factor
     all,amazon_ses: amazon_ses
-    all,postal: cryptography
+    all,postal: postal
 setenv =
     # tell runtests.py to limit some test tags based on extras factor
     none: ANYMAIL_SKIP_TESTS=amazon_ses,postal