CLI hangs when no arguments provided

[layday]

Perhaps foolishly, I expected validate-pyproject to auto-detect my pyproject.toml inside the current working directory; but pipx run validate-pyproject[all] hangs indefinitely, presumably waiting for stdin (line 219 of cli.py).

[abravalheri]

Hi @layday, thank you very much for pointing that out.

Probably it should make clear to the user that it is going to use the stdin.