Stop unconditionally importing distutils

distutils is fully deprecated and will stop working in 3.12, so just
stop the unconditional import. The conditional one remains, and that
favours setuptools. Everything still works the same.

setup.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-import distutils
 import platform
 try:
   from setuptools import setup, Extension