SparkPost: drop support for multiple from_email (#213)

SparkPost's API no longer allows this, and now returns
a confusing error message about return_path.

(Not treating as a breaking change in Anymail, because
the breaking change was in the SparkPost API. This just
improves the error message in the unlikely event anyone
is trying to use this feature.)

Closes #212

anymail/backends/sparkpost.py

@@ -117,10 +117,8 @@ class SparkPostPayload(RequestsPayload):
             "recipients": [],
         }
 
-    def set_from_email_list(self, emails):
-        # SparkPost supports multiple From email addresses,
-        # as a single comma-separated string
-        self.data["content"]["from"] = ", ".join(email.address for email in emails)
+    def set_from_email(self, email):
+        self.data["content"]["from"] = email.address
 
     def set_to(self, emails):
         if emails: