setup: drop legacy osx compiler tuning to enable universal builds This was triggering deprecation warnings about migrating to `packaging.version` from `distutils` Version classes with `make local`. But rather than migrate that code, let's just get rid of some ~10-12 year old workarounds. As a bonus, the cext libraries that are built are now universal binaries containing x86_64 and arm64 images (at least when built on macOS 11.4 with Xcode 12.5 and the universal version of Python 3.9.13). Several things to note here: - Apple dropped support for 10.15 in Nov 2022, and OS X Lion that is referenced is 10.7 (unsupported since late 2014) - `xcode4` was basically always True because of the `>=` check (10.8 used Xcode 5, and I have Xcode 10.2 on 10.14) - `xcode51` was always False for modern-ish Xcode, because of the exact version string matching - Python 3.8 only supports OS X 10.9+; the Python 3.9.1+ universal installer is macOS 11+ only, and Python 3.10 drops the x86_64 installer to deliver only the universal installer. All of this is to say, the only thing lost by dropping this code on modern Xcode is that `os.environ['ARCHFLAGS'] = ''` is no longer set. But we probably shouldn't be setting that anymore, as shown by the universal libraries now being generated. I was able to `make local` and `python3 run-tests.py --local` with python 3.9.9, Xcode 10.2, and macOS 10.14.6, and didn't incur any more than the usual few test errors, so this should still work on some older versions of macOS.