Create 'localize_metadata' function to normalize the outputs for .met…

…adata.

importlib_metadata/__init__.py

@@ -34,6 +34,7 @@
     NullFinder,
     install,
     localize_dist,
+    localize_metadata,
 )
 from ._functools import apply, method_cache, pass_none
 from ._itertools import always_iterable, bucket, unique_everseen
@@ -477,6 +478,7 @@ def _discover_resolvers():
         return filter(None, declared)
 
     @property
+    @apply(localize_metadata)
     def metadata(self) -> _meta.PackageMetadata:
         """Return the parsed metadata for this Distribution.
 

importlib_metadata/_compat.py

@@ -1,12 +1,13 @@
 from __future__ import annotations
 
+import email.message
 import importlib.metadata
 import platform
 import sys
 import warnings
 from typing import cast
 
-import importlib_metadata
+import importlib_metadata._adapters
 
 
 __all__ = ['install', 'NullFinder']
@@ -76,3 +77,17 @@ def localize_dist(
         return importlib_metadata.PathDistribution(dist._path)
     warnings.warn(f"Unrecognized distribution subclass {dist.__class__}")
     return cast(importlib_metadata.Distribution, dist)
+
+
+if sys.version_info >= (3, 10):
+    StdlibMessage = importlib.metadata._adapters.Message
+else:
+    StdlibMessage = email.message.Message
+
+
+def localize_metadata(
+    input: importlib_metadata._adapters.Message | StdlibMessage,
+) -> importlib_metadata.Message:
+    if isinstance(input, importlib_metadata._adapters.Message):
+        return input
+    return importlib_metadata._adapters.Message(input)