Add precise values for enum members where possible (#11299)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
Co-authored-by: Alex Waygood <alex.waygood@gmail.com>

stubs/pika/pika/delivery_mode.pyi

@@ -1,5 +1,5 @@
 from enum import Enum
 
 class DeliveryMode(Enum):
-    Transient: int
-    Persistent: int
+    Transient = 1
+    Persistent = 2

stubs/pika/pika/exchange_type.pyi

@@ -1,7 +1,7 @@
 from enum import Enum
 
 class ExchangeType(Enum):
-    direct: str
-    fanout: str
-    headers: str
-    topic: str
+    direct = "direct"
+    fanout = "fanout"
+    headers = "headers"
+    topic = "topic"