Fix CoilSnake's build

[PhoenixBound]

Here we go again... there's been a lot of confusion here so I want to double-check everything and make sure we get it done right, so nothing has to be fixed again for a long time.

This PR seeks to eliminate deprecated parts of the build system and clean up the build instructions.

Recent pushes in the Python ecosystem have led to the standardization of different build systems (setuptools, poetry, flit, whatever else) to share a common metadata format, a pyproject.toml file, in order to make installing dependencies more robust (for build dependencies) and less dependent on setuptools (and on its predecessor distutils, which was removed from the standard library in Python 3.12). As part of this, setuptools has been trying to eliminate most places that directly run setup.py from the shell, in favor of using the standard tools. They've also deprecated and attempted to remove many features that are non-standard, including running test suites.

Having a setup.py file itself isn't deprecated. In fact, as of creating this PR in August, CoilSnake couldn't delete setup.py, because pyproject.toml only supported configuring extension modules in setup.py until a few weeks later (when v74.1.0 released). Only certain uses and fields in the file are deprecated. Despite that, in the current version of this PR, I moved everything I could to the new standardized format (except the extension modules, since the syntax is experimental). I'm not aware of any plans for most of those things to be deprecated, so I'm fine with moving things like the package name back to setup.py to keep the PR minimal.

This PR's a draft for now. Some scattered ideas for what might come next, or might not:

• Fix version info with setuptools-scm, because it probably doesn't work right now
• Figure out appropriate reliable changes to the spec file will allow Pyinstaller to work on multiple Python versions. (Nowadays eggs seem to include the name of the CPython implementation and use a version number with no . in it. What's the pattern?)
• See if automating building the EXE (with a custom build backend or otherwise) is possible/worthwhile
• Maybe offer a "wheel"-format download option...? (aspirational)
  • Use the stable/limited ABI wherever possible so that one wheel can be used with many Python versions, hopefully making pyinstaller specs easier to write too
    • So far it seems like several changes are needed. Even seemingly basic functions like PyList_Check and PyLong_Check aren't part of the stable ABI, and various ergonomics issues around type names in error messages exist that aren't solved until Python 3.13.
  • Change compression extension modules to be optional? It wouldn't do anything now, but once ccscript is replaced with a pure-Python module, we could have a nearly pure-Python version of CoilSnake (very portable)
  • Research ways to cross-compile extension modules for other platforms? Test building wheels for other platforms using crossenv?
  • This would also require updating the CCScript and CCScriptWriter dependencies to do the same thing
• If Pyinstaller doesn't work out, check out this incomplete page of alternate recommendations