Select legacy GCM or actual FCM endpoint for send based on the gcm_key (#110)

* Select legacy GCM or actual FCM endpoint for send based on the key provided.

* Fixed flake8 issue

Author: kilgoretrout1985

pywebpush/__init__.py

@@ -308,7 +308,13 @@ def send(self, data=None, headers=None, ttl=0, gcm_key=None, reg_id=None,
                 'content-encoding': content_encoding,
             })
         if gcm_key:
-            endpoint = 'https://android.googleapis.com/gcm/send'
+            # guess if it is a legacy GCM project key or actual FCM key
+            # gcm keys are all about 40 chars (use 100 for confidence),
+            # fcm keys are 153-175 chars
+            if len(gcm_key) < 100:
+                endpoint = 'https://android.googleapis.com/gcm/send'
+            else:
+                endpoint = 'https://fcm.googleapis.com/fcm/send'
             reg_ids = []
             if not reg_id:
                 reg_id = self.subscription_info['endpoint'].rsplit('/', 1)[-1]