Only allow splatting iterables if function takes at least one positional arg

Bug Report

At runtime the following code snippet:

def f(*, x: int = 9, y:int = 8) -> None:
    pass

kwargs = {"x": 1, "y": 2}
f(*kwargs)

results in TypeError: f() takes 0 positional arguments but 2 were given. However, mypy does not report any issues.

To Reproduce

1. Execute python interpreter for the code snipped above.
2. Run mypy for the code snippet above.

Expected Behavior

Mypy should yield an error. For example pytype yields the following error:

Function f expects 0 arg(s), got 2 [wrong-arg-count]
         Expected: (*, x, y)
  Actually passed: (_, _)

Actual Behavior

Success: no issues found in 1 source file

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: None. Simply mypy <FILENAME>.
• Mypy configuration options from mypy.ini (and other config files): N/A (defaults)
• Python version used: 3.8.2
• Operating system and version: Mac OS Big Sur

Si inizi eseguendo il Python reproducer e il comando mypy descritti nell'issue, quindi si tracci il modo in cui mypy controlla uno starred iterable passato a una funzione con soli keyword-only parameters. Il lavoro è completato quando mypy segnala un errore sul numero di argomenti per questo esempio e il comportamento è coperto da un test di regressione.