Fix mypyc build errors on master

The latest release of `types-setuptools` is causing type-check errors (and, therefore, build errors) on the `master` branch: see, e.g. https://github.com/python/mypy/actions/runs/4275505309/jobs/7442902732. python#14781 addressed some of the new errors, but didn't quite fix the build.

mypyc/build.py

@@ -63,13 +63,14 @@ def get_extension() -> type[Extension]:
     # We can work with either setuptools or distutils, and pick setuptools
     # if it has been imported.
     use_setuptools = "setuptools" in sys.modules
+    extension_class: type[Extension]
 
     if not use_setuptools:
-        from distutils.core import Extension
+        from distutils.core import Extension as extension_class
     else:
-        from setuptools import Extension
+        from setuptools import Extension as extension_class
 
-    return Extension
+    return extension_class
 
 
 def setup_mypycify_vars() -> None: