Include backend name in requests user-agent string

Author: medmunds

anymail/backends/base_requests.py

@@ -35,8 +35,9 @@ def open(self):
             if not self.fail_silently:
                 raise
         else:
-            self.session.headers["User-Agent"] = "Anymail/%s %s" % (
-                __version__, self.session.headers.get("User-Agent", ""))
+            self.session.headers["User-Agent"] = "django-anymail/{version}-{esp} {orig}".format(
+                esp=self.esp_name.lower(), version=__version__,
+                orig=self.session.headers.get("User-Agent", ""))
             return True
 
     def close(self):