Add module level entrypoint

[pohmelie]

I was surprised when discover that httpx can't be invoked as cli via python -m httpx, only as an registered entrypoint as httpx. The first one is everywhere in python world: ruff, mypy, black, pip, pytest, etc.

[lovelydinosaur]

Okay, yeah this could be valid.

Looking at flask/__main__.py and httpx/__init__.py I think we'd need this?...

main.py...

try:
    from ._main import main
except ImportError:  # pragma: no cover

    def main() -> None:  # type: ignore
        import sys

        print(
            "The httpx command line client could not run because the required "
            "dependencies were not installed.\nMake sure you've installed "
            "everything with: pip install 'httpx[cli]'"
        )
        sys.exit(1)

main()

[pohmelie]

It looks like «yes»