Cleanup: Avoid Python 3.7 deprecation warning on 'async' keyword

Fixes #92

anymail/backends/mandrill.py

@@ -252,9 +252,9 @@ class MandrillPayload(RequestsPayload):
         ('template_content', combine, None),
     )
 
-    def set_async(self, async):
+    def set_async(self, is_async):
         self.deprecated_to_esp_extra('async')
-        self.esp_extra['async'] = async
+        self.esp_extra['async'] = is_async
 
     def set_ip_pool(self, ip_pool):
         self.deprecated_to_esp_extra('ip_pool')

tests/test_mandrill_djrill_features.py

@@ -13,7 +13,12 @@ class MandrillBackendDjrillFeatureTests(MandrillBackendMockAPITestCase):
     # These features should now be accessed through esp_extra
 
     def test_async(self):
-        self.message.async = True
+        # async becomes a keyword in Python 3.7. If you have code like this:
+        #   self.message.async = True
+        # it should be changed to:
+        #   self.message.esp_extra = {"async": True}
+        # (The setattr below keeps these tests compatible, but isn't recommended for your code.)
+        setattr(self.message, 'async', True)  # don't do this; use esp_extra instead
         with self.assertWarnsRegex(DeprecationWarning, 'async'):
             self.message.send()
         data = self.get_api_call_json()