Update packaging version to install Python 3.10 wheels.

[domdfcoding]

What's the problem this feature will solve?
Currently it is not possible to install wheels for Python 3.10 because the current version of packaging vendored with pip thinks the tag should be 3_10 when it should actually be 310. The relevant entry from pip debug is cp3_10-cp3_10-linux_x86_64.

An example of a wheel I'm trying to install is numpy-1.19.2-cp310-cp310-linux_x86_64.whl, which was built with wheel 0.36.2

Describe the solution you'd like
A fix (pypa/packaging#355) has been merged into packaging, and a new release (20.8) has been made with that fix. Would it be possible to have a new release of pip with the latest version of packaging vendored?

Alternative Solutions
I have manually applied to changes from pypa/packaging#355 to a local copy of pip, and have been able to successfully install the wheel. The relevant entry from pip debug is then cp310-cp310-linux_x86_64.

Additional context

• PEP 641, which was rejected in favour of keeping 310 as the tag.
• Wheel has already been updated to support building wheels with 310 tags (Fails to build wheel for Python 3.10 wheel#354 and pypa/wheel@f6fd247).

[uranusjr]

I feel we should include this asap, not until 21.0.

[pradyunsg]

I'll do the update after lunch -- if someone wants to file a PR updating packaging in pip before that, please do and let me know you're doing that! :)