[issue39648] Update math.gcd() to accept "n" arguments.

[Mark Dickinson]

Mark Dickinson <dickinsm at gmail.com> added the comment:

@Ananthakrishnan: Sure, go ahead.

Let's make the process a bit more streamlined than last time around: see if you can put together a PR that passes all the CI checks before asking for review. If you get stuck, make a work-in-progress PR and ask for help in a comment on that PR rather than via email.

A couple of pointers:

- Take a look at the existing math.hypot implementation to see how to deal with multiple arguments. (Note that argument clinic doesn't currently work for *args functions.)

- For the first working version, don't bother making special cases for the zero-argument or one-argument gcd (or even the case of two arguments). You just want the equivalent of the following Python code, which is perfectly general:

    def gcd(*args):
        g = 0
        for arg in args:
            g = gcd_2arg(g, operator.index(arg))
        return g

where gcd_2arg is the original 2-argument gcd (in C, _PyLong_GCD). We can always optimise later.

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