Introduce a configuration option type

* Normalise rebuild values
* Ensure that the tuple interface continues to work

tests/test_config.py

@@ -1,5 +1,5 @@
 """Test the sphinx.config.Config class."""
-
+import pickle
 import time
 from pathlib import Path
 from unittest import mock
@@ -7,10 +7,24 @@ from unittest import mock
 import pytest
 
 import sphinx
-from sphinx.config import ENUM, Config, check_confval_types
+from sphinx.config import ENUM, Config, _Opt, check_confval_types
+from sphinx.deprecation import RemovedInSphinx90Warning
 from sphinx.errors import ConfigError, ExtensionError, VersionRequirementError
 
 
+def test_config_opt_deprecated(recwarn):
+    opt = _Opt('default', '', ())
+
+    with pytest.warns(RemovedInSphinx90Warning):
+        default, rebuild, valid_types = opt
+
+    with pytest.warns(RemovedInSphinx90Warning):
+        _ = opt[0]
+
+    with pytest.warns(RemovedInSphinx90Warning):
+        _ = list(opt)
+
+
 @pytest.mark.sphinx(testroot='config', confoverrides={
     'root_doc': 'root',
     'nonexisting_value': 'True',
@@ -71,6 +85,16 @@ def test_config_not_found(tmp_path):
         Config.read(tmp_path)
 
 
+@pytest.mark.parametrize("protocol", list(range(pickle.HIGHEST_PROTOCOL)))
+def test_config_pickle_protocol(tmp_path, protocol: int):
+    config = Config()
+
+    pickled_config = pickle.loads(pickle.dumps(config, protocol))
+
+    assert list(config.values) == list(pickled_config.values)
+    assert repr(config) == repr(pickled_config)
+
+
 def test_extension_values():
     config = Config()