simplify retry logic & API (#2)

Author: eli-darkly

ld_eventsource/__init__.py

@@ -1,4 +1,6 @@
 from ld_eventsource.errors import *
 from ld_eventsource.event import *
 from ld_eventsource.request_params import *
+from ld_eventsource.retry_delay_strategy import *
+from ld_eventsource.retry_filter import *
 from ld_eventsource.sse_client import *