gh-141510: Change repr(frozendict) for empty dict

[vstinner]

repr(frozendict()) returns "frozendict()" instead of "frozendict({})".

• Issue: PEP 814: Add built-in frozendict type #141510

[vstinner]

For example, frozenset has a similar formatting:

>>> frozenset()  # empty
frozenset()
>>> frozenset("a")
frozenset({'a'})

[corona10]

lgtm