gh-117215 Make the fromskey() signature match dict.fromkeys().

[rhettinger]

This minor edit improves tooltips and help() without actually changing the API (everything that used to work still does and everything that didn't used to work still doesn't).

>>> help(dict.fromkeys)
fromkeys(iterable, value=None, /) class method of builtins.dict
    Create a new dictionary with keys from iterable and values set to value.

>>> help(ChainMap.fromkeys)
fromkeys(iterable, value=None, /) class method of collections.ChainMap
    Create a new ChainMap with keys from thhe iterable and values set to value.

• Issue: collection.ChainMap.fromkeys argument 'value' has incorrect argument kind #117215