gh-128810: Added .shorthand property to IPv4Network and IPv6Network in the ipaddress module

[jfuruness]

Pretty straightforward. Addresses gh-128810. Added a .shorthand property to all ip_networks for ease-of-use, since this is how they are commonly written by network operators. This way for a prefix 1.2.0.0/16 the shorthand of 1.2/16 can easily be accessed.

#128810

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[rruuaanng]

Suggested change

	Added a .shorthand function to IPv4Network and IPv6Network's in the ipaddress module to be able to display a prefix such as 1.2.0.0/16 as it's shorthand version (often used by network operators) as 1.2/16.
	Add the :attr:`!shorthand` property to :class:`~ipaddress.IPv4Network` and :class:`~ipaddress.IPv6Network` in the :mod:`ipaddress` module to display a prefix such as ``1.2.0.0/16`` as its shorthand version (often used by network operators), like ``1.2/16``.

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[ZeroIntensity]

We're still discussing approaches in the issue, and if we want to actually go for this (the RFC specifying this is a best practice rather than a standard).

That, and I don't like shorthand being the name, because that's not really what it's called universally. Something like cidr or cidr_shorthand would be more correct.

[jfuruness]

I will update the PR shortly, still discussing the desired behavior as ZeroIntensity mentioned. Thank you all for your feedback!