setup.py: Import distutils directly instead of through setuptools

In setuptools 65.6.0 the vendored distutils package changed to relative
imports. Importing `setuptools._distutils` would import an unpatched
version of distutils, while importing `distutils` directly imports a
monkeypatched version that works.

Reported upstream at pypa/setuptools#3743 and
the problem was investigated by Anderson Bravalheri.

Closes: alan-turing-institute#77

setup.py

@@ -8,7 +8,7 @@
 from setuptools import Extension
 from setuptools import find_packages
 from setuptools import setup
-from setuptools._distutils.core import Command
+from distutils.core import Command
 
 # Package meta-data.
 AUTHOR = "Gertjan van den Burg"