rename: store state in scratch

This PR changes RenameCodemod to store its per-module state in `self.context.scratch` which gets properly reset between files.

libcst/codemod/commands/rename.py

@@ -92,14 +92,38 @@ def __init__(self, context: CodemodContext, old_name: str, new_name: str) -> Non
         self.old_module: str = old_module
         self.old_mod_or_obj: str = old_mod_or_obj
 
-        self.as_name: Optional[Tuple[str, str]] = None
-
-        # A set of nodes that have been renamed to help with the cleanup of now potentially unused
-        # imports, during import cleanup in `leave_Module`.
-        self.scheduled_removals: Set[cst.CSTNode] = set()
-        # If an import has been renamed while inside an `Import` or `ImportFrom` node, we want to flag
-        # this so that we do not end up with two of the same import.
-        self.bypass_import = False
+    @property
+    def as_name(self) -> Optional[Tuple[str, str]]:
+        if "as_name" not in self.context.scratch:
+            self.context.scratch["as_name"] = None
+        return self.context.scratch["as_name"]
+
+    @as_name.setter
+    def as_name(self, value: Optional[Tuple[str, str]]) -> None:
+        self.context.scratch["as_name"] = value
+
+    @property
+    def scheduled_removals(self) -> Set[cst.CSTNode]:
+        """A set of nodes that have been renamed to help with the cleanup of now potentially unused
+        imports, during import cleanup in `leave_Module`."""
+        if "scheduled_removals" not in self.context.scratch:
+            self.context.scratch["scheduled_removals"] = set()
+        return self.context.scratch["scheduled_removals"]
+
+    @scheduled_removals.setter
+    def scheduled_removals(self, value: Set[cst.CSTNode]) -> None:
+        self.context.scratch["scheduled_removals"] = value
+
+    @property
+    def bypass_import(self) -> bool:
+        """A flag to indicate that an import has been renamed while inside an `Import` or `ImportFrom` node."""
+        if "bypass_import" not in self.context.scratch:
+            self.context.scratch["bypass_import"] = False
+        return self.context.scratch["bypass_import"]
+
+    @bypass_import.setter
+    def bypass_import(self, value: bool) -> None:
+        self.context.scratch["bypass_import"] = value
 
     def visit_Import(self, node: cst.Import) -> None:
         for import_alias in node.names: