_toolchain.cxx: use distutils from setuptools.

The distutils module from the standard library is deprecated and will
be removed in Python 3.12, and PEP 632 recommends using
distutils.ccompiler from setuptools, instead.

This code should eventually be rewritten to use zig-pypi, but for now
this suffices.

nmigen/_toolchain/cxx.py

@@ -1,7 +1,7 @@
 import tempfile
 import sysconfig
 import os.path
-from distutils import ccompiler
+from setuptools import distutils
 
 
 __all__ = ["build_cxx"]
@@ -17,7 +17,7 @@ def build_cxx(*, cxx_sources, output_name, include_dirs, macros):
         # the output directory directly.
         os.chdir(build_dir.name)
 
-        cc_driver = ccompiler.new_compiler()
+        cc_driver = distutils.ccompiler.new_compiler()
         cc_driver.output_dir = "."
 
         cc = sysconfig.get_config_var("CC")