sepp: fix build with Python 3.12+ (distutils removal)

[gonzabotmdp]

Summary

sepp's setup.py fails under Python 3.12+ with:

File ".../distribute_setup.py", line 25, in <module>
    from distutils import log
ModuleNotFoundError: No module named 'distutils'

Two separate issues in setup.py, both from the same root cause (distutils removed from the stdlib in 3.12+):

1. from distribute_setup import use_setuptools + use_setuptools(version="0.6.24") -- a 2010-era helper script (from back when setuptools/distribute were separate forks, merged back together in 2013) that bootstraps installing setuptools if it's missing. Completely obsolete: setuptools has shipped by default for well over a decade, and this package already unconditionally does from setuptools import find_packages two lines below. Dropped entirely.
2. from distutils.core import setup, Command -- setuptools.Command is a documented drop-in for distutils.core.Command (the ConfigSepp subclass's own docstring literally says "setuptools Command"), and setuptools.setup for distutils.core.setup. Switched the import.

Testing

Installed sepp@4.5.1 (a busco@6.1.0 dependency) end to end with Python 3.14, confirmed the install phase completes and produces working binaries.