Always raise warnings for deprecated feature checks

[radarhere]

When I run the test suite without WebP installed, it fails.

Tests/test_features.py::test_check FAILED [ 2%] ================================== FAILURES =================================== _________________________________ test_check __________________________________ def test_check() -> None: # Check the correctness of the convenience function for module in features.modules: assert features.check_module(module) == features.check(module) for codec in features.codecs: assert features.check_codec(codec) == features.check(codec) for feature in features.features: if "webp" in feature: > with pytest.warns(DeprecationWarning): E Failed: DID NOT WARN. No warnings of type (<class 'DeprecationWarning'>,) were emitted. E Emitted warnings: []. 

The backstory of this is that #8213 deprecated features.check("transp_webp"), features.check("webp_mux") and features.check("webp_anim").

However, the deprecation warning is only raised if the underlying WebP module is installed, and the missing module doesn't raise an error earlier than the warning.

Pillow/src/PIL/features.py

Lines 149 to 159 in e93dcc1

	try:
	imported_module = __import__(module, fromlist=["PIL"])
	if isinstance(flag, bool):
	deprecate(f'check_feature("{feature}")', 12)
	return flag
	return getattr(imported_module, flag)
	except ModuleNotFoundError:
	return None
	except ImportError as ex:
	warnings.warn(str(ex))
	return None

This PR updates features.py so that the feature checks raise a deprecation warning when WebP is not installed as well.