Modernize python scripts packaging, python versioning

[bollwyvl]

Hi folks, thanks for the antlrverse!

The current use of scripts in the PythonX runtimes makes it harder to generate otherwise-solid cross-platform downstream packages that have different constraints than e.g. PyPI. Adopting a newer approach, would look like:

    entry_points={
        'console_scripts': [
            "pygrun=some-importable.some-module:some-method"
        ]
    },

and would means that...

• the functionality would move either into antlr4-pythonX-runtime.pygrun:main or some other thing
• the .bat wrapper script would no longer be needed
• downstreams could reason more about the generated files and where to put them
  • selfishly, for conda-forge, this would mean going from a matrix of 20+ builds to a single build.

Furthermore, as the description suggests a bottom python pin, formally declaring this would further improve the metadata for both pip users and others.

    python_requires='>=3.7',

Thanks again!

[bollwyvl]

We went ahead and did this over on conda-forge:

conda-forge/antlr-python-runtime-feedstock#31

Will submit a patch with that approach.

[bollwyvl]

Opened #3379