Update cibuildwheel config to stop building Python 3.9 and free-threaded wheels (#19154)

.github/workflows/release-artifacts.yml

@@ -150,12 +150,14 @@ jobs:
       - name: Build wheels
         run: python -m cibuildwheel --output-dir wheelhouse
         env:
-          # Skip testing for platforms which various libraries don't have wheels
-          # for, and so need extra build deps.
+          # The platforms that we build for are determined by the
+          # `tool.cibuildwheel.skip` option in `pyproject.toml`.
+
+          # We skip testing wheels for the following platforms in CI:
           #
-          # cp39-*: Python 3.9 is EOL.
-          # cp3??t-*: Free-threaded builds are not currently supported.
-          CIBW_TEST_SKIP: pp3*-* cp39-* cp3??t-* *i686* *musl*
+          # pp3*-* (PyPy wheels) broke in CI (TODO: investigate).
+          # musl: (TODO: investigate).
+          CIBW_TEST_SKIP: pp3*-* *musl*
 
       - uses: actions/upload-artifact@330a01c490aca151604b8cf639adc76d48f6c5d4 # v5.0.0
         with:

changelog.d/19154.misc

@@ -0,0 +1 @@
+Properly stop building wheels for Python 3.9 and free-threaded CPython.

pyproject.toml

@@ -392,13 +392,23 @@ build-backend = "poetry.core.masonry.api"
 
 [tool.cibuildwheel]
 # Skip unsupported platforms (by us or by Rust).
-# See https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip for the list of build targets.
+#
+# See https://cibuildwheel.readthedocs.io/en/stable/options/#build-skip for the
+# list of supported build targets.
+#
+# Also see `.github/workflows/release-artifacts.yml` for the list of
+# architectures we build for (based on the runner OS types we use), as well as
+# the platforms we exclude from testing in CI.
+#
 # We skip:
-#  - CPython 3.8: EOLed
-#  - musllinux i686: excluded to reduce number of wheels we build.
-#    c.f. https://github.com/matrix-org/synapse/pull/12595#discussion_r963107677
-skip = "cp38* *-musllinux_i686"
-# Enable non-default builds.
+#  - free-threaded cpython builds: these are not currently supported.
+#  - cp38: Python 3.8 is end-of-life.
+#  - cp39: Python 3.9 is end-of-life.
+#  - i686: We don't support 32-bit platforms.
+skip = "cp3??t-* cp38-* cp39-* *i686*"
+# Enable non-default builds. See the list of available options:
+# https://cibuildwheel.pypa.io/en/stable/options#enable
+#
 # "pypy" used to be included by default up until cibuildwheel 3.
 enable = "pypy"