Minor clean up (#362)

refactor: add __all__

Author: kayjan

bigtree/tree/export/_yield_tree.py

@@ -11,6 +11,12 @@
 from bigtree.tree.helper import get_subtree
 from bigtree.utils import constants, iterators
 
+__all__ = [
+    "YieldTree",
+    "HYieldTree",
+    "VYieldTree",
+]
+
 T = TypeVar("T", bound=node.Node)
 TStyle = TypeVar(
     "TStyle",