Include extras when formatting a url direct reference

[richafrank]

As specified by the grammar in https://www.python.org/dev/peps/pep-0508/ :

url_req       = name wsp* extras? wsp* urlspec wsp+ quoted_marker?
urlspec       = '@' wsp* <URI_reference>
extras        = '[' wsp* extras_list? wsp* ']'

For #1518 , if folks agree with the change.

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• Provided the tests for the changes.
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[AndydeCleyre]

I'm on my phone right now and can't be sure, but this might also be sitting in #1329 which had to touch on this.

[richafrank]

@AndydeCleyre Yep, I see it! (FWIW you don't need the generator expression.) Sounds like we're in agreement on the behavior. Feel free to use yours instead, unless you want to pull this in sooner.

[AndydeCleyre]

Thanks, I'll nix the pointless generator and merge latest / rebase within a day.

I don't want to merge #1329 until @atugushev clears it; he can decide whether to merge this one beforehand.

[atugushev]

Superseded by #1582.