When the builtins module was converted to Argument Clinic, round() was omitted because existing signature of round() wasn't supported with the inspect module. Now round() supports None as the value for the ndigits arguments, and this is the default value. round() now has the signature round(number, ndigits=None).

Proposed PR converts round() to Argument Clinic. This has two benefits:

1. inspect.signature() now works with round.

2. Faster parsing keyword arguments.