Allow Private :: Do Not Upload classifier.

This is a special case because it is not listed in a trove classifier
but it is a way to make sure that a private package is not get uploaded
on PyPI by accident.

Implementation on PyPI side:
    pypi/warehouse#5440
Issue about officially documenting the trick:
    pypa/packaging.python.org#643

flit/validate.py

@@ -13,6 +13,12 @@
 
 log = logging.getLogger(__name__)
 
+CUSTOM_CLASSIFIERS = frozenset({
+    # https://github.com/pypa/warehouse/pull/5440
+    'Private :: Do Not Upload',
+})
+
+
 def get_cache_dir() -> Path:
     """Locate a platform-appropriate cache directory for flit to use
 
@@ -96,6 +102,7 @@ def validate_classifiers(classifiers):
     classifiers = set(classifiers)
     try:
         valid_classifiers = _read_classifiers_cached()
+        valid_classifiers.update(CUSTOM_CLASSIFIERS)
         problems = _verify_classifiers(classifiers, valid_classifiers)
     except (FileNotFoundError, PermissionError) as e1:
         # We haven't yet got the classifiers cached or couldn't read it
@@ -120,8 +127,8 @@ def validate_classifiers(classifiers):
         log.warning(
             "Couldn't get list of valid classifiers to check against")
         return problems
-    else:
-        return _verify_classifiers(classifiers, valid_classifiers)
+    valid_classifiers.update(CUSTOM_CLASSIFIERS)
+    return _verify_classifiers(classifiers, valid_classifiers)
 
 
 def validate_entrypoints(entrypoints):

tests/test_validate.py

@@ -145,6 +145,26 @@ def mock_get_cache_dir():
     assert classifiers == {"A", "B", "C"}
 
 
+def test_validate_classifiers_private(monkeypatch):
+    """
+    Test that `Private :: Do Not Upload` considered a valid classifier.
+    This is a special case because it is not listed in a trove classifier
+    but it is a way to make sure that a private package is not get uploaded
+    on PyPI by accident.
+
+    Implementation on PyPI side:
+        https://github.com/pypa/warehouse/pull/5440
+    Issue about officially documenting the trick:
+        https://github.com/pypa/packaging.python.org/issues/643
+    """
+    monkeypatch.setattr(fv, "_read_classifiers_cached", lambda: set())
+
+    actual = fv.validate_classifiers({'invalid'})
+    assert actual == ["Unrecognised classifier: 'invalid'"]
+
+    assert fv.validate_classifiers({'Private :: Do Not Upload'}) == []
+
+
 @responses.activate
 @pytest.mark.parametrize("error", [PermissionError, OSError(errno.EROFS, "")])
 def test_download_and_cache_classifiers_with_unacessible_dir(monkeypatch, error):