Add extlinks alias for PyPI

[sinoroc]

extlinks = {
    # ...
    "pypi": ("https://pypi.org/project/%s", "%s"),
}

I think this could be real helpful.

Originally posted by @chrysle in https://github.com/pypa/packaging.python.org/pull/1362#discussion_r1387128645

[webknjaz]

Per https://github.com/pypa/pip/pull/12616, integrating the latest sphinx-issues version can replace this.

[shenxianpeng]

AFAIK the latest sphinx-issues does not support gh, so can't fully replace extlinks like https://github.com/pypa/pip/pull/12616

https://github.com/pypa/packaging.python.org/blob/5a04ac85a59917c7da5542515277690376254aef/source/conf.py#L137