Fix dunder-method positional-only parameter discrepancies in third-party stubs (#14529)

stubs/pika/pika/exchange_type.pyi

@@ -1,7 +1,19 @@
-from enum import Enum
+import sys
 
-class ExchangeType(Enum):
-    direct = "direct"
-    fanout = "fanout"
-    headers = "headers"
-    topic = "topic"
+if sys.version_info >= (3, 11):
+    from enum import StrEnum
+
+    class ExchangeType(StrEnum):
+        direct = "direct"
+        fanout = "fanout"
+        headers = "headers"
+        topic = "topic"
+
+else:
+    from enum import Enum
+
+    class ExchangeType(str, Enum):
+        direct = "direct"
+        fanout = "fanout"
+        headers = "headers"
+        topic = "topic"