Revert "Use an enum for sentinel"

This reverts commit 35771f643d.

src/_pytest/assertion/rewrite.py

@@ -14,7 +14,6 @@ import sys
 import tokenize
 import types
 from collections import defaultdict
-from enum import Enum
 from pathlib import Path
 from pathlib import PurePath
 from typing import Callable
@@ -54,11 +53,7 @@ PYTEST_TAG = f"{sys.implementation.cache_tag}-pytest-{version}"
 PYC_EXT = ".py" + (__debug__ and "c" or "o")
 PYC_TAIL = "." + PYTEST_TAG + PYC_EXT
 
-
-class ScopeEndMarkerType(Enum):
-    """Special marker that denotes we have just left a function or class definition."""
-
-    ScopeEndMarker = 1
+_SCOPE_END_MARKER = object()
 
 
 class AssertionRewritingHook(importlib.abc.MetaPathFinder, importlib.abc.Loader):
@@ -733,13 +728,13 @@ class AssertionRewriter(ast.NodeVisitor):
 
         # Collect asserts.
         self.scope = (mod,)
-        nodes: List[Union[ast.AST, ScopeEndMarkerType]] = [mod]
+        nodes: List[Union[ast.AST, object]] = [mod]
         while nodes:
             node = nodes.pop()
             if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef, ast.ClassDef)):
                 self.scope = tuple((*self.scope, node))
-                nodes.append(ScopeEndMarkerType.ScopeEndMarker)
-            if node is ScopeEndMarkerType.ScopeEndMarker:
+                nodes.append(_SCOPE_END_MARKER)
+            if node == _SCOPE_END_MARKER:
                 self.scope = self.scope[:-1]
                 continue
             assert isinstance(node, ast.AST)