Add support for pip>=21.3

[atugushev]

Changes:

• fix removed in pypa/pip@d051a00 pip._internal.utils.misc.get_installed_distributions used in piptools.scripts.sync

TODO:

• merge Bump pip minimum version to >= 21.2 #1500
• rebase/merge-master

Contributor checklist

• Provided the tests for the changes.
• Assure PR title is short, clear, and good to be included in the user-oriented changelog

Maintainer checklist

• Assure one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

Fixes #1503

[atugushev]

@AndydeCleyre I've prepared pip-tools for the upcoming pip-21.3 release (due date - 6:30pm UK time on Monday, Oct 11). Kindly requesting some assistance with review/merge TODOs 🙏🏻

[atugushev]

@jazzband/pip-tools now this is all set up and ready for review.

[atugushev]

Thanks @ssbarnea 🙏🏻

[ssbarnea]

Nopb. I was bit confused by the fact the pip version bumping was merged before that change but no problem. I hope to see this released ASAP as I do not want to have to update deps to avoid errors (or pin down pip).

[atugushev]

@ssbarnea release is on the way #1504

[AndydeCleyre]

I know I'm late, but importing typing directly (and later using typing.Union rather than just Union) seems unnecessary.

[atugushev]

Thank you 👍🏻

[rrauenza]

thanks for the super quick fix, everyone! Saw this breaking across my cicd's and I was beginning to pin pip version...