Show License-Expression if present in package metadata

With Core Metadata 2.4 a new field, License-Expression, has been added.
If it's present, favor it over the deprecated (with PEP 639) legacy
unstructured License field.

Closes: #13112

Author: befeleme

news/13112.feature.rst

@@ -0,0 +1 @@
+Prefer to display ``License-Expression`` in ``pip show`` if metadata version is at least 2.4.

src/pip/_internal/commands/show.py

@@ -66,6 +66,7 @@ class _PackageInfo(NamedTuple):
     author: str
     author_email: str
     license: str
+    license_expression: str
     entry_points: List[str]
     files: Optional[List[str]]
 
@@ -161,6 +162,7 @@ def _get_requiring_packages(current_dist: BaseDistribution) -> Iterator[str]:
             author=metadata.get("Author", ""),
             author_email=metadata.get("Author-email", ""),
             license=metadata.get("License", ""),
+            license_expression=metadata.get("License-Expression", ""),
             entry_points=entry_points,
             files=files,
         )
@@ -186,7 +188,10 @@ def print_results(
         write_output("Home-page: %s", dist.homepage)
         write_output("Author: %s", dist.author)
         write_output("Author-email: %s", dist.author_email)
-        write_output("License: %s", dist.license)
+        if dist.metadata_version >= "2.4" and dist.license_expression:
+            write_output("License-Expression: %s", dist.license_expression)
+        else:
+            write_output("License: %s", dist.license)
         write_output("Location: %s", dist.location)
         if dist.editable_project_location is not None:
             write_output(