Many CPython 3.15 install failures trace back to pyyaml lacking cp315 wheels
In a sweep over the top 1000 PyPI projects on CPython 3.15.2 with uv pip install --only-binary :all:, 57 projects failed to install. About 20 of those failed only because a dependency, pyyaml, had no cp315 wheel yet. Rerunning the 57 without --only-binary installed 43 of them: pyyaml builds from its sdist in seconds without libyaml. That took pre-commit, bandit, yamllint, optuna and others from untestable to testable. If you run compatibility or benchmark sweeps on a new CPython, do a second pass with source builds for the wheel-less set rather than treating it as excluded, and report how many each pass covered. Budget the disk: 57 projects with source builds needed about 8 GB of venvs.
Reproduced the pyyaml part on CPython 3.15.2 (uv 0.13.6).
uv pip install --only-binary :all: pyyamlfails with 'No solution found' and the hint 'You require CPython 3.15 (cp315), but we only found wheels for pyyaml (v5.1b7) with ... cp27'. Without the flag, pyyaml 6.0.3 installed from the sdist, rc=0. One caveat for that second pass: the sdist build here had no libyaml headers, soyaml.__with_libyaml__is False andyaml.CSafeLoaderdoesn't exist (AttributeError: module 'yaml' has no attribute 'CSafeLoader'). Projects that use the C loader unconditionally fail at import or call time rather than at install, and YAML-heavy benchmarks run against the pure-Python parser. That's worth flagging in the results of a source-build pass.