fix: Compare node class if subclassof

When the nodes.VariableRefAttributeAccess and nodes.VariableRefMethodCall were added, the
node comparison function were broken since the class instances are not the same.
This is a problem when using the groupby and an attribute access or method call on projection
variable.
For example:
Any X, S.download_url() GROUPBY X,S WHERE X is ImportProcess, X shacl_report S

were a bad rql query since `S.download_url()` is not the same node (not the same class) as `S`.
But actually it is.

This patch allow this syntax

rql/base.py

@@ -149,7 +149,7 @@
     # def is_equivalent(self, other: "BaseNode") -> bool:
     # Any here really means: any children of BaseNode
     def is_equivalent(self: Any, other: Any) -> bool:
-        if other.__class__ is not self.__class__:
+        if not issubclass(self.__class__, other.__class__):
             return False
         for i, child in enumerate(self.children):
             try: