Drop support for Python 3.7

[chrysle]

Drop support for Python 3.7 which is EOL in the end of June.

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).

[atugushev]

Bump version here also, please:

pip-tools/.pre-commit-config.yaml

Lines 11 to 15 in b05acb1

	- repo: https://github.com/asottile/pyupgrade
	rev: v3.3.1
	hooks:
	- id: pyupgrade
	args: [--py37-plus]

[chrysle]

@atugushev Thank you, done. Maybe a list with files relevant when dropping a version can be added to the contributing guide to ease maintenance.

[webknjaz]

Should we wait until the end of July, then? I don't think it costs much to merge this later, right?

[chrysle]

Sounds like a good suggestion.

[chrysle]

Yesterday was the last day of support.

[webknjaz]

I wonder if a release could be made right before this is merged..

[atugushev]

I'll prepare a release shortly.

[atugushev]

FYI: the release 6.14.0 is blocked by #1863 (comment). Release drafter now suggests 7.0.0 release.

[webknjaz]

@chrysle plz add git revert 3fdee98 to this.

[atugushev]

Ready to go 👍🏻