include Symbol in rsa package, ignore empty __init__ files with Ruff

Author: bygu4

pyformlang/rsa/__init__.py

@@ -9,6 +9,8 @@
     A recursive automaton.
 :class:`pyformlang.rsa.Box`:
     A constituent part of a recursive automaton.
+:class:`pyformlang.rsa.Symbol`:
+    A nonterminal of the Box.
 
 References
 ----------
@@ -20,10 +22,11 @@
 """
 
 from .recursive_automaton import RecursiveAutomaton
-from .box import Box
+from .box import Box, Symbol
 
 
 __all__ = [
     "RecursiveAutomaton",
     "Box",
+    "Symbol",
 ]

ruff.toml

@@ -44,6 +44,7 @@ select = [
 ]
 
 ignore = [
+    "D104",
     "D416",
     "ANN401",
 ]