Upgrade packaging to 21.0

[lkollar]

Closes: #10089

[uranusjr]

Grr, that news file bot.

[lkollar]

Looks like Python 3.9 on Windows encountered the same singal-related failure in test_download_http_url__no_directory_traversal and test_prepare_download__log:

signalnum = <Signals.SIGINT: 2>
handler = <bound method InterruptibleMixin.handle_sigint of <pip._internal.cli.progress_bars.DownloadProgressSpinner object at 0x000002BCDC661130>>

    @_wraps(_signal.signal)
    def signal(signalnum, handler):
>       handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
E       ValueError: signal only works in main thread of the main interpreter

Doesn't seem to have anything to do with packaging 🤷

[puetzk]

This also fixes #8649 and #9962. These issues were already closed with "R: wrong project" since the issue was pypa/packaging#396, but taking packaging 21.0 means pip picks that change up and thus "fixes" pip in the end-user sense (pip installs the right wheels again).

[pradyunsg]

Third time's the charm?