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 #1362 (comment)

[webknjaz]

Per pypa/pip#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 pypa/pip#12616

packaging.python.org/source/conf.py

Line 137 in 5a04ac8

"gh": (f"{github_url}/%s", "GitHub: %s"),