Upgrade packaging to 20.3

[xuhdev]

cc @pradyunsg @bdwyer2

[xuhdev]

Any chance to get this reviewed :) @pradyunsg @chrahunt
@xavfernandez

[uranusjr]

Is there a particular pressing need for this upgrade? I believe it is generally preferred to upgrade vendor packages before a new pip release instead to avoid repetitive PRs.

[uranusjr]

(To clarify, the PR itself LGTM.)

[sbidoul]

Note we have at least one bug that depends on an open packaging PR: pypa/packaging#264

[xavfernandez]

The PR looks good to me but I see no reason to bump the version now, this will be done prior to releasing pip 20.1.

[pradyunsg]

I have no issues with waiting. Let's just keep this open for a bit; until April. :)

[chrahunt]

A few benefits of not waiting:

1. if the new release introduces any bugs or incompatibilities, we're more likely to catch them
2. compatibility is ensured with each PR instead of one PR near the release date - potentially less work later

[BrownTruck]

Hello!

I am an automated bot and I have noticed that this pull request is not currently able to be merged. If you are able to either merge the master branch into this pull request or rebase this pull request against master then it will be eligible for code review and hopefully merging!

[xuhdev]

This has already been done in 2694496 . Probably no need to merge this one. Thanks!