fix: add __instancecheck__ and __subclasscheck__ to _DeprecatedProxy

[Borda]

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

isinstance(x, proxy) and issubclass(Sub, proxy) raised TypeError when a deprecated_class() proxy was used as the second argument, because _DeprecatedProxy is an instance, not a type.

Add instancecheck and subclasscheck instance methods that delegate to the active class (target if set, otherwise wrapped obj). Neither method emits a deprecation warning — type checks are structural and must not consume the warning budget or produce noise in loops.

Part of #109

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[dosubot]

Documentation Updates

1 document(s) were updated by changes in this PR:

README

View Changes

@@ -563,6 +563,9 @@
 access and forwards attribute, item, and call operations to the replacement class.
 Use `args_mapping` to rename or drop kwargs when the deprecated class is called.
 
+> [!NOTE]
+> Type checks with `isinstance()` and `issubclass()` work transparently with `@deprecated_class` proxies and do not emit deprecation warnings, as these are structural checks rather than actual usage of the deprecated API.
+
 ```python
 from enum import Enum
 from dataclasses import dataclass

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[Copilot]

Pull request overview

Fixes isinstance(x, proxy) / issubclass(Sub, proxy) behavior when deprecated_class() returns an _DeprecatedProxy (an instance), by adding type-protocol hooks intended to delegate the checks to the active class without emitting deprecation warnings.

Changes:

• Add __instancecheck__ and __subclasscheck__ to _DeprecatedProxy to support isinstance/issubclass when a proxy is used as the RHS.
• Add unit tests covering isinstance / issubclass delegation and the “no warnings during type checks” expectation (partially).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/deprecate/proxy.py	Implements __instancecheck__ / __subclasscheck__ delegation for proxies.
tests/unittests/test_proxy.py	Adds new unit tests for the type-protocol behavior.