Explicitly define printable representation

In Python 3.12 the implementation of OrderedDict.__repr__ changed such
that ExpiringDict.__repr__ may trigger expiration and raise KeyError. To
avoid this, explicitly implement ExpiringDict.__repr__ using validated
ExpiringDict.items().

Resolves mailgun#55

expiringdict/__init__.py

@@ -85,6 +85,9 @@ def __getitem__(self, key, with_age=False):
                 del self[key]
                 raise KeyError(key)
 
+    def __repr__(self):
+        return f"{self.__class__.__name__}({self.items()})"
+
     def __setitem__(self, key, value, set_time=None):
         """ Set d[key] to value. """
         with self.lock: