Fix test vector generators to work with Python 3.12

[jtraglia]

When generating test vectors, we noticed that the action no longer worked. Most of the issues were related to the recent switch to Python 3.12. There were a few other small issues we fixed & improvements we made too though.

• Change runner from size-chungus-x64 to size-xl-x64.
  • According to devops: "they don't have capacity to fire up a chungus-x64 atm."
• Fix input name for reference, the variable is defined as ref at the top of the file.
• Add new build_wheel which builds the pyspec wheel.
  • Instead of building and installing this for each generator, just do it once.
  • When building this several times in parallel, there was a race condition which caused issues.
  • Lots of "no such file or directory" issues, as it was packing & deleting at the same time.
  • Oh and this uses the build module to do this, which is setuptool's replacement.
• Call build_wheel once before running the generators.
• Fix package data includes & reformat for consistency.
  • When building the wheel, it was unable to find some files because they weren't being included.
  • Stuff like *.yaml needed to be **/*.yaml to recursively collect all yaml files.
• Update python-snappy to the latest version which provides a pre-built wheel for Python 3.12.
  • Without this, you get "snappy-c.h: No such file or directory" errors.
• Add setuptools and pytest to generator dependencies & delete individual requirements files.
  • In my opinion, there's no sense in keeping these if they are all the same.