Support disabling trove classifier verification

Add a HATCH_NO_VERIFY_TROVE_CLASSIFIERS hook to disable verification
of trove classifiers.  This makes it possible to build newer packages
against old versions of trove-classifiers (or even without the package
installed).  This is a convenience feature for Linux distributions
that do not want to track minimum required trove-classifiers versions
as required by various packages.

Fixes #1368

backend/src/hatchling/metadata/core.py

@@ -977,8 +977,6 @@ def classifiers(self) -> list[str]:
         if self._classifiers is None:
             import bisect
 
-            import trove_classifiers
-
             if 'classifiers' in self.config:
                 classifiers = self.config['classifiers']
                 if 'classifiers' in self.dynamic:
@@ -994,23 +992,36 @@ def classifiers(self) -> list[str]:
                 message = 'Field `project.classifiers` must be an array'
                 raise TypeError(message)
 
-            known_classifiers = trove_classifiers.classifiers | self._extra_classifiers
+            verify_classifiers = not os.environ.get("HATCH_NO_VERIFY_TROVE_CLASSIFIERS")
+            if verify_classifiers:
+                import trove_classifiers
+
+                known_classifiers = trove_classifiers.classifiers | self._extra_classifiers
+                sorted_classifiers = list(trove_classifiers.sorted_classifiers)
+
+                for classifier in sorted(self._extra_classifiers - trove_classifiers.classifiers):
+                    bisect.insort(sorted_classifiers, classifier)
+
             unique_classifiers = set()
 
             for i, classifier in enumerate(classifiers, 1):
                 if not isinstance(classifier, str):
                     message = f'Classifier #{i} of field `project.classifiers` must be a string'
                     raise TypeError(message)
 
-                if not self.__classifier_is_private(classifier) and classifier not in known_classifiers:
+                if not self.__classifier_is_private(classifier) and verify_classifiers and classifier not in known_classifiers:
                     message = f'Unknown classifier in field `project.classifiers`: {classifier}'
                     raise ValueError(message)
 
                 unique_classifiers.add(classifier)
 
-            sorted_classifiers = list(trove_classifiers.sorted_classifiers)
-            for classifier in sorted(self._extra_classifiers - trove_classifiers.classifiers):
-                bisect.insort(sorted_classifiers, classifier)
+            if not verify_classifiers:
+                import re
+
+                # combined text-numeric sort that ensures that Python versions sort correctly
+                split_re = re.compile(r'(\D*)(\d*)')
+                sort_key = lambda value: [(a, int(b) if b else None) for a, b in split_re.findall(value)]
+                sorted_classifiers = sorted(classifiers, key=sort_key)
 
             self._classifiers = sorted(
                 unique_classifiers, key=lambda c: -1 if self.__classifier_is_private(c) else sorted_classifiers.index(c)

tests/backend/metadata/test_core.py

@@ -950,12 +950,37 @@ def test_entry_not_string(self, isolation):
         with pytest.raises(TypeError, match='Classifier #1 of field `project.classifiers` must be a string'):
             _ = metadata.core.classifiers
 
-    def test_entry_unknown(self, isolation):
+    def test_entry_unknown(self, isolation, monkeypatch):
+        monkeypatch.delenv("HATCH_NO_VERIFY_TROVE_CLASSIFIERS", False)
         metadata = ProjectMetadata(str(isolation), None, {'project': {'classifiers': ['foo']}})
 
         with pytest.raises(ValueError, match='Unknown classifier in field `project.classifiers`: foo'):
             _ = metadata.core.classifiers
 
+    def test_entry_unknown_no_verify(self, isolation, monkeypatch):
+        monkeypatch.setenv("HATCH_NO_VERIFY_TROVE_CLASSIFIERS", "1")
+        classifiers = [
+            'Programming Language :: Python :: 3.11',
+            'Programming Language :: Python :: 3.11',
+            'Programming Language :: Python :: 3.9',
+            'Development Status :: 4 - Beta',
+            'Private :: Do Not Upload',
+            'Foo',
+        ]
+        metadata = ProjectMetadata(str(isolation), None, {'project': {'classifiers': classifiers}})
+
+        assert (
+            metadata.core.classifiers
+            == metadata.core.classifiers
+            == [
+                'Private :: Do Not Upload',
+                'Development Status :: 4 - Beta',
+                'Foo',
+                'Programming Language :: Python :: 3.9',
+                'Programming Language :: Python :: 3.11',
+            ]
+        )
+
     def test_correct(self, isolation):
         classifiers = [
             'Programming Language :: Python :: 3.11',