Fix exception class spacing.

Author: Lukasa

hyper/http20/exceptions.py

@@ -25,13 +25,15 @@ class HPACKDecodingError(HTTP20Error):
     """
     pass
 
+
 class ConnectionError(HTTP20Error):
     """
     The remote party signalled an error affecting the entire HTTP/2
     connection, and the connection has been closed.
     """
     pass
 
+
 class ProtocolError(HTTP20Error):
     """
     The remote party violated the HTTP/2 protocol.