Unpin the types-setuptools build dependency (python#14787)

- The latest release of `types-setuptools` started causing type-check
errors (and, therefore, mypyc 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.
- python#14788 then pinned the `types-setuptools` dependency as a temporary
stopgap measure.
- This PR now attempts to unpin `types-setuptools` and fix the build
errors in a more principled way.

Author: AlexWaygood

mypyc/build.py

@@ -51,7 +51,7 @@
 
 try:
     # Import setuptools so that it monkey-patch overrides distutils
-    import setuptools  # noqa: F401
+    import setuptools
 except ImportError:
     if sys.version_info >= (3, 12):
         # Raise on Python 3.12, since distutils will go away forever
@@ -63,13 +63,16 @@ 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
+        import distutils.core
+
+        extension_class = distutils.core.Extension
     else:
-        from setuptools import Extension
+        extension_class = setuptools.Extension
 
-    return Extension
+    return extension_class
 
 
 def setup_mypycify_vars() -> None: