Add casts for package metadata

In the latest mypy (0.942), importlib.metadata.metadata now returns
a Protocol named PackageMetadata that doesn't implement the get
method.  The actual object remains a Message class, however, so the
get method works correctly.  Only mypy checking fails.

Pending python/typeshed#7767, which may
reveal that it is intentional that get not be supported, cast the
return value to Message so that the existing functions work.  This
seems better than adding try/catch blocks for every piece of
metadata of interest given that the omission of get appears to be
unintentional.

src/safir/metadata.py

@@ -5,7 +5,7 @@
 
 from email.message import Message
 from importlib.metadata import metadata
-from typing import Optional
+from typing import Optional, cast
 
 from pydantic import BaseModel, Field
 
@@ -66,7 +66,7 @@ def get_metadata(*, package_name: str, application_name: str) -> Metadata:
     project_urls, Source code
         Used as the ``respository_url``.
     """
-    pkg_metadata: Message = metadata(package_name)
+    pkg_metadata: Message = cast(Message, metadata(package_name))
     return Metadata(
         name=application_name,
         version=pkg_metadata.get("Version", "0.0.0"),

tests/metadata_test.py

@@ -3,20 +3,18 @@
 
 from __future__ import annotations
 
+from email.message import Message
 from importlib.metadata import metadata
-from typing import TYPE_CHECKING
+from typing import cast
 
 import pytest
 
 from safir.metadata import get_metadata, get_project_url
 
-if TYPE_CHECKING:
-    from email.message import Message
-
 
 @pytest.fixture(scope="session")
 def safir_metadata() -> Message:
-    return metadata("safir")
+    return cast(Message, metadata("safir"))
 
 
 def test_get_project_url(safir_metadata: Message) -> None: