chore(testing): add 3.12-dev testing

[ZStriker19]

A new weblog variant was added for system-tests here: DataDog/system-tests#1617
This runs that new variant so we test 2.x on 3.12

Checklist

• Change(s) are motivated and described in the PR description.
• Testing strategy is described if automated tests are not included in the PR.
• Risk is outlined (performance impact, potential for breakage, maintainability, etc).
• Change is maintainable (easy to change, telemetry, documentation).
• Library release note guidelines are followed. If no release note is required, add label changelog/no-changelog.
• Documentation is included (in-code, generated user docs, public corp docs).
• Backport labels are set (if applicable)

Reviewer Checklist

• Title is accurate.
• No unnecessary changes are introduced.
• Description motivates each change.
• Avoids breaking API changes unless absolutely necessary.
• Testing strategy adequately addresses listed risk(s).
• Change is maintainable (easy to change, telemetry, documentation).
• Release note makes sense to a user of the library.
• Reviewer has explicitly acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment.
• Backport labels are set in a manner that is consistent with the release branch maintenance policy
• If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-09-25 19:50:57

Comparing candidate commit 6a67196 in PR branch zachg/add_system-tests_for_2.x_and_312 with baseline commit 4997cab in branch 2.x.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 90 metrics, 0 unstable metrics.

[ZStriker19]

I think parametric is just failing because we're not skipping the deprecated b3 tests, which this should fix: DataDog/system-tests#1621