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>

stdlib/asyncio/constants.pyi

@@ -15,6 +15,6 @@ if sys.version_info >= (3, 12):
     THREAD_JOIN_TIMEOUT: Literal[300]
 
 class _SendfileMode(enum.Enum):
-    UNSUPPORTED: int
-    TRY_NATIVE: int
-    FALLBACK: int
+    UNSUPPORTED = 1
+    TRY_NATIVE = 2
+    FALLBACK = 3