tree: _level should be an instance invariable

_level is initialized as a class variable
whereas it should be an instance variable
of DtshBuiltinTree.

src/dtsh/builtin_tree.py

@@ -108,7 +108,7 @@ class DtshBuiltinTree(DtshCommand):
 """
 
     # Maximum display depth, 0 to follow all non disabled nodes.
-    _level: int = 0
+    _level: int
 
     def __init__(self, shell: Dtsh):
         super().__init__(
@@ -121,6 +121,7 @@ def __init__(self, shell: Dtsh):
             ]
         )
         self._dtsh = shell
+        self._level = 0
 
     @property
     def usage(self) -> str: