Postmark: add track_clicks support

Closes #38

tests/test_postmark_backend.py

@@ -325,8 +325,15 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
 
     def test_track_clicks(self):
         self.message.track_clicks = True
-        with self.assertRaisesMessage(AnymailUnsupportedFeature, 'track_clicks'):
-            self.message.send()
+        self.message.send()
+        data = self.get_api_call_json()
+        self.assertEqual(data['TrackLinks'], 'HtmlAndText')
+
+        # Also explicit "None" for False (to override server default)
+        self.message.track_clicks = False
+        self.message.send()
+        data = self.get_api_call_json()
+        self.assertEqual(data['TrackLinks'], 'None')
 
     def test_template(self):
         self.message.template_id = 1234567
@@ -369,6 +376,7 @@ class PostmarkBackendAnymailFeatureTests(PostmarkBackendMockAPITestCase):
         self.assertNotIn('TemplateId', data)
         self.assertNotIn('TemplateModel', data)
         self.assertNotIn('TrackOpens', data)
+        self.assertNotIn('TrackLinks', data)
 
     def test_esp_extra(self):
         self.message.esp_extra = {

tests/test_postmark_integration.py

@@ -53,6 +53,7 @@ class PostmarkBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
             # no metadata, send_at, track_clicks support
             tags=["tag 1"],  # max one tag
             track_opens=True,
+            track_clicks=True,
         )
         message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
         message.attach("attachment2.csv", "ID,Name\n1,Amy Lina", "text/csv")