fix!: Replace Python 2-style ABC patterns with Python 3 equivalents

[jsonbailey]

Summary

• Replace __metaclass__ = ABCMeta class-body assignments (Python 2-only, silently ignored in Python 3) with proper (ABC) base class inheritance across 4 modules and 11 classes.
• Replace @abstractproperty decorators (deprecated since Python 3.3) with the correct @property + @abstractmethod stacking in 7 locations across 4 files.
• Remove now-unused ABCMeta and abstractproperty from all from abc import ... lines.

Files changed

File	Changes
ldclient/interfaces.py	11 classes: __metaclass__ → (ABC); 5 @abstractproperty → @property + @abstractmethod
ldclient/hook.py	Hook class + metadata property
ldclient/plugin.py	Plugin class
ldclient/migrations/migrator.py	Migrator class
ldclient/testing/integrations/persistent_feature_store_test_base.py	tester_class property
ldclient/testing/integrations/big_segment_store_test_base.py	tester_class property

Why

In Python 3, __metaclass__ = ABCMeta inside a class body is a no-op — the metaclass is never applied, so @abstractmethod decorators were not enforced. Using (ABC) as a base class is the correct Python 3 pattern and actually enforces the abstract interface.

@abstractproperty has been deprecated since Python 3.3 and emits DeprecationWarning in newer Python versions. The correct replacement is to stack @property on top of @abstractmethod.

Test plan

• make test — 1005 passed, 194 skipped (database integration tests, expected)

🤖 Generated with Claude Code

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Note

Medium Risk
Marked breaking because stricter ABC enforcement can surface at runtime if custom subclasses omitted required abstract members; behavior of complete implementations should be unchanged.

Overview
Replaces legacy Python 2 abstract-base patterns with idiomatic Python 3 across SDK interfaces, hooks, plugins, migrations, and integration test bases.

__metaclass__ = ABCMeta is removed in favor of inheriting (ABC) on classes such as FeatureStore, Hook, Plugin, Migrator, and related ldclient.interfaces types, so @abstractmethod is actually enforced at instantiation time. @abstractproperty is replaced with @property + @abstractmethod on abstract properties like Hook.metadata, FeatureStore.initialized, and test-base tester_class. Imports drop unused ABCMeta / abstractproperty.

Contract test PostingHook now implements metadata so it stays a valid Hook subclass under the stricter ABC rules.

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Tracked Internally: SDK-2463