chore: remove Python 3.7 support

[Saransh-cpp]

Following #348 (comment), this PR removes the support of Python 3.7 from vector. I'm still not completely sure if there was a concensus regarding this but I'll be happy to close this PR if we do want to support Python 3.7.

Description

Kindly take a look at CONTRIBUTING.md.

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[henryiii]

Ruff's config still has:

target-version = "py37"

[Saransh-cpp]

Oops, fixed it, thank you!

[henryiii]

@jpivarski thoughts? Should this go in 1.1? I’m thinking it might be good to do now since 1.1 doesn’t support numba on 3.7, while 1.0 does.

[jpivarski]

Yes, it could be good to do all of the dependency version threshold changes in 1.1, instead of using multiple minor versions or using up numbers.

It's a tighter set of version constraints for someone who wants #347, but it doesn't sound like a likely scenario. ("I want ak.sum to work on Vectors, but I'm using Python 3.7.")

I'd say go ahead and add it to Vector 1.1.

[henryiii]

It's not just about using up numbers, this release is actually a downgrade for Python 3.7 users - they now can't use numba with vector anymore if we don't bump the Python version now. Using ak.sum is a new feature and won't break existing code like removing numba could.