Improve the repr() of `_marker` (#174)

src/typing_extensions.py

diff --git a/src/typing_extensions.py b/src/typing_extensions.py
index 30255a0..a30c01b 100644
--- a/src/typing_extensions.py
+++ b/src/typing_extensions.py
@@ -95,7 +95,13 @@
 # The functions below are modified copies of typing internal helpers.
 # They are needed by _ProtocolMeta and they provide support for PEP 646.
 
-_marker = object()
+
+class _Sentinel:
+    def __repr__(self):
+        return "<sentinel>"
+
+
+_marker = _Sentinel()
 
 
 def _check_generic(cls, parameters, elen=_marker):