chore: make cythonize incremental by default

[P403n1x87]

We make the cythonisation of pyx sources incremental by default. The old behaviour whereby all Cython sources are forcibly re-transpiled can be recovered by setting DD_SETUP_FORCE_CYTHONIZE=1.

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has 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

[github-actions]

CODEOWNERS have been resolved as:

setup.py                                                                @DataDog/python-guild

[github-actions]

Bootstrap import analysis

Comparison of import times between this PR and base.

Summary

The average import time from this PR is: 274 ± 2 ms.

The average import time from base is: 278 ± 3 ms.

The import time difference between this PR and base is: -4.3 ± 0.1 ms.

Import time breakdown

The following import paths have shrunk:

ddtrace.auto 2.178 ms (0.79%)

ddtrace.bootstrap.sitecustomize 1.503 ms (0.55%)

ddtrace.bootstrap.preload 1.503 ms (0.55%)

ddtrace.internal.remoteconfig.client 0.671 ms (0.24%)

ddtrace 0.675 ms (0.25%)

ddtrace.internal._unpatched 0.032 ms (0.01%)

json 0.032 ms (0.01%)

json.decoder 0.032 ms (0.01%)

re 0.032 ms (0.01%)

enum 0.032 ms (0.01%)

types 0.032 ms (0.01%)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-16 15:02:15

Comparing candidate commit a1ad396 in PR branch chore/incremental-cythonize with baseline commit 5692392 in branch main.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 412 metrics, 3 unstable metrics.

scenario:iastaspectssplit-splitlines_aspect

• 🟥 execution_time [+104.001ns; +133.565ns] or [+7.023%; +9.019%]