[Python APIView] update setup.py to pyproject.toml + fix message on py runtime errors

[swathipil]

1. Update setup.py to pyproject.toml for both apiview-stub-generator and apistubgentest packages. Updating apistubgentest will require you to modify the apiview_test.py that checks that both packages that install setup.py and pyproject.toml can be parsed:

• make pyproject.toml the default
• make setup.py the one that requires copying over to a temp directory to run parse on.

2. If you have a package with a python-requires higher that your current environment runtime, I would expect you get an error from pip back, that it cannot install the package. I'd like to catch the error, and express it as "This package requires a higher runtime tham the current one, please install at least Python 3.XX".

Point is:
If the runtime is enough, just make it work
If the runtime is not enough, explain what runtime is needed
Problem it's solving, is I don't want to require everyone to have 3.14 installed "just in case", but I don't want to prevent someone with a 3.12+ package to get an APIView. This allows the parser to adapt.