harmonize with documentation

although inconsistent (see Requires-External, Provides-Extra, Provides-Dist, Obsoletes-Dist...) pluralize the result for multiple use of `License-File`

https://packaging.pypa.io/en/stable/metadata.html#packaging.metadata.RawMetadata

> Any core metadata field that can be specified multiple times or can hold multiple values in a single field have a key with a plural name.

src/packaging/metadata.py

@@ -133,7 +133,7 @@ class RawMetadata(TypedDict, total=False):
 
     # Metadata 2.4 - PEP 639
     license_expression: str
-    license_file: list[str]
+    license_files: list[str]
 
 
 _STRING_FIELDS = {
@@ -157,7 +157,7 @@ class RawMetadata(TypedDict, total=False):
 _LIST_FIELDS = {
     "classifiers",
     "dynamic",
-    "license_file",
+    "license_files",
     "obsoletes",
     "obsoletes_dist",
     "platforms",
@@ -261,7 +261,7 @@ def _get_payload(msg: email.message.Message, source: bytes | str) -> str:
     "keywords": "keywords",
     "license": "license",
     "license-expression": "license_expression",
-    "license-file": "license_file",
+    "license-file": "license_files",
     "maintainer": "maintainer",
     "maintainer-email": "maintainer_email",
     "metadata-version": "metadata_version",
@@ -655,7 +655,7 @@ def _process_license_expression(self, value: str) -> str:
                 f"{value!r} is invalid for {{field}}", cause=exc
             ) from exc
 
-    def _process_license_file(self, value: list[str]) -> list[str]:
+    def _process_license_files(self, value: list[str]) -> list[str]:
         paths = []
         for path in value:
             if ".." in path:
@@ -805,7 +805,7 @@ def from_email(cls, data: bytes | str, *, validate: bool = True) -> Metadata:
     """:external:ref:`core-metadata-license`"""
     license_expression: _Validator[str | None] = _Validator(added="2.4")
     """:external:ref:`core-metadata-license-expression`"""
-    license_file: _Validator[list[str] | None] = _Validator(added="2.4")
+    license_files: _Validator[list[str] | None] = _Validator(added="2.4")
     """:external:ref:`core-metadata-license-file`"""
     classifiers: _Validator[list[str] | None] = _Validator(added="1.1")
     """:external:ref:`core-metadata-classifier`"""

tests/test_metadata.py

@@ -216,7 +216,7 @@ def test_complete(self):
             "        to redistribute it."
         )
         assert raw["license_expression"] == "Apache-2.0 OR BSD-2-Clause"
-        assert raw["license_file"] == ["LICENSE.APACHE", "LICENSE.BSD"]
+        assert raw["license_files"] == ["LICENSE.APACHE", "LICENSE.BSD"]
         assert raw["classifiers"] == [
             "Development Status :: 4 - Beta",
             "Environment :: Console (Text Based)",
@@ -638,32 +638,32 @@ def test_invalid_license_expression(self):
             meta.license_expression  # noqa: B018
 
     @pytest.mark.parametrize(
-        "license_file",
+        "license_files",
         [
             ["LICENSE.txt", "licenses/*"],
             [],
             ["licenses/LICENSE.MIT", "licenses/LICENSE.CC0"],
             ["LICENSE"],
         ],
     )
-    def test_valid_license_file(self, license_file):
+    def test_valid_license_files(self, license_files):
         meta = metadata.Metadata.from_raw(
-            {"license_file": license_file}, validate=False
+            {"license_files": license_files}, validate=False
         )
-        assert meta.license_file == license_file
+        assert meta.license_files == license_files
 
     @pytest.mark.parametrize(
-        "license_file",
+        "license_files",
         [
             ["../LICENSE"],
             ["./../LICENSE"],
             ["licenses/../LICENSE"],
         ],
     )
-    def test_invalid_license_file(self, license_file):
+    def test_invalid_license_files(self, license_files):
         meta = metadata.Metadata.from_raw(
-            {"license_file": license_file}, validate=False
+            {"license_files": license_files}, validate=False
         )
 
         with pytest.raises(metadata.InvalidMetadata):
-            meta.license_file  # noqa: B018
+             meta.license_files  # noqa: B018