IPv4Interface.is_loopback ignores the network prefix, unlike IPv6Interface

Bug report

Bug description:

IPv4Interface does not override is_loopback, so it inherits IPv4Address.is_loopback and only checks the host address, ignoring the network. IPv6Interface, on the other hand, checks both the address and the network:

>>> import ipaddress
>>> ipaddress.IPv4Interface('127.0.0.1/4').is_loopback
True
>>> ipaddress.IPv4Interface('127.0.0.1/4').network.is_loopback
False
>>> ipaddress.IPv6Interface('::1/64').is_loopback
False

127.0.0.1/4 has a network of 112.0.0.0/4, which is not within the loopback range 127.0.0.0/8, yet is_loopback returns True.

This is the same class of inconsistency already fixed for is_unspecified in gh-115766: an interface property should consider both the address and its network. IPv6Interface.is_loopback already does this:

@property
def is_loopback(self):
    return super().is_loopback and self.network.is_loopback

IPv4Interface should get the same override for consistency.

Note this is a behavior change (127.0.0.1/4 etc. would change from True to False).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-151786

Start at IPv4Interface.is_loopback and compare it with the IPv6Interface implementation shown in the issue. Check the existing ipaddress tests and ensure an interface considers both its address and network, including the documented 127.0.0.1/4 behavior.