fix: typo on setuptools (#11432)

Author: Mr-Sunglasses

stubs/setuptools/setuptools/_distutils/ccompiler.pyi

@@ -16,7 +16,7 @@ def show_compilers() -> None: ...
 
 class CCompiler:
     src_extensions: ClassVar[list[str] | None]
-    obj_extensions: ClassVar[str | None]
+    obj_extension: ClassVar[str | None]
     static_lib_extension: ClassVar[str | None]
     shared_lib_extension: ClassVar[str | None]
     static_lib_format: ClassVar[str | None]