fix: restore CONSTANTS singleton identity across pickle/deepcopy

[derek73]

Summary

Closes #169.

A default HumanName shares the module-level CONSTANTS singleton as its .C. pickle and copy.deepcopy cannot preserve object identity, so without custom hooks .C was serialized by value — causing has_own_config to flip to True and bloating every serialized default name with a full Constants copy (~1000+ entries).

Fix

Add __getstate__/__setstate__ to HumanName:

• __getstate__ replaces .C with None as a sentinel when it is the shared CONSTANTS singleton.
• __setstate__ restores the CONSTANTS reference from that sentinel.

None is a safe sentinel because HumanName.__init__ always replaces a None constants argument with a fresh Constants(), so .C is never legitimately None at rest. Both pickle and copy.deepcopy call these hooks, so one implementation covers both.

Instance-config names (where .C is not CONSTANTS) are unaffected — their own Constants copy continues to round-trip through the existing __dict__ path established in #168.

Tests

• test_pickle_default_name_preserves_singleton_identity — C is CONSTANTS and has_own_config == False after a pickle round-trip.
• test_deepcopy_default_name_preserves_singleton_identity — same assertions after copy.deepcopy.

Also adds assertIs/assertIsNot shims to HumanNameTestBase (the custom test base class that replaced unittest.TestCase) and updates three existing assertTrue(x is ...) callsites to use them for clearer failure messages.

Full suite: 710 passed, 20 xfailed.

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This was prepared with the assistance of AI, under my direction and review.

[derek73]

Post-review follow-up (3c3c04f): strengthened the test suite based on a comprehensive review of this PR.

What was added:

• Parsed-field assertions in test_pickle_default_name_preserves_singleton_identity and test_deepcopy_default_name_preserves_singleton_identity — both tests now also assert str(restored) == str(hn), .first, and .last match, so a future __setstate__ refactor that accidentally drops state would be caught immediately rather than only the singleton check failing.

• test_pickle_instance_config_name_preserves_own_config — closes the one gap in the four-quadrant matrix ({default config, own config} × {pickle, deepcopy}). The deepcopy side already had test_name_instance_deepcopy_isolates_instance_config; pickle now has a counterpart that asserts has_own_config stays True and C is not CONSTANTS after a round-trip.

• test_shallow_copy_default_name_preserves_singleton_identity — documents that copy.copy preserves CONSTANTS identity by sharing the reference directly (shallow copy copies __dict__ without calling __getstate__/__setstate__), so no hook is needed. Pins the contract so it's visible if that behavior ever changes.

76 tests pass, no regressions.