Add pendings in _BfxWebsocketBucket. Add new logic for selecting the bucket with less connections. Add ._unsubscribe coroutine.

bfxapi/websocket/errors.py

@@ -1,5 +1,6 @@
 __all__ = [
     "ConnectionNotOpen",
+    "TooManySubscriptions",
     "WebsocketAuthenticationRequired",
     "InvalidAuthenticationCredentials",
     "EventNotSupported",
@@ -20,6 +21,13 @@ class ConnectionNotOpen(BfxWebsocketException):
 
     pass
 
+class TooManySubscriptions(BfxWebsocketException):
+    """
+    This error indicates an attempt to subscribe to a public channel after reaching the limit of simultaneous connections.
+    """
+
+    pass
+
 class WebsocketAuthenticationRequired(BfxWebsocketException):
     """
     This error indicates an attempt to access a protected resource without logging in first.