fix: support nested parens in license expressions (#931)

* support nested parens

* format

---------

Co-authored-by: Henry Schreiner <[email protected]>

Author: Liam-DeVoe

src/packaging/licenses/__init__.py

@@ -90,7 +90,11 @@ def canonicalize_license_expression(
             python_tokens.append("False")
         elif token == "with":
             python_tokens.append("or")
-        elif token == "(" and python_tokens and python_tokens[-1] not in {"or", "and"}:
+        elif (
+            token == "("
+            and python_tokens
+            and python_tokens[-1] not in {"or", "and", "("}
+        ):
             message = f"Invalid license expression: {raw_license_expression!r}"
             raise InvalidLicenseExpression(message)
         elif token == ")" and python_tokens and python_tokens[-1] == "(":

tests/test_metadata.py

@@ -698,6 +698,12 @@ def test_optional_defaults_to_none(self, field_name):
                 "(LicenseRef-Special-License OR licenseref-OtherLicense) OR unlicense",
                 "(LicenseRef-Special-License OR LicenseRef-OtherLicense) OR Unlicense",
             ),
+            # we don't canonicalize redundant parens, instead leaving them as-is
+            # in the license expression.
+            ("(MIT)", "(MIT)"),
+            ("((MIT))", "((MIT))"),
+            ("(( MIT ))", "((MIT))"),
+            ("((MIT AND (MIT)))", "((MIT AND (MIT)))"),
         ],
     )
     def test_valid_license_expression(self, license_expression, expected):