Build and publish cp315/cp315t wheels (#26)

Bump cibuildwheel v3.3.0 -> v4.2.1, which builds CPython 3.15 by default
(3.15.0rc2, ABI-stable), and add 3.15 and 3.15t to the wheel matrix.
NumPy 2.5.2+ ships cp315/cp315t wheels, so build isolation needs no
source build. Without cp315 wheels, cvxpy's windows/3.15 CI falls back to
the sdist, which can't find OpenBLAS without vcpkg.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: Transurgeon

.github/workflows/build-and-publish.yml

@@ -31,9 +31,9 @@ jobs:
       fail-fast: false
       matrix:
         os: [ ubuntu-22.04, macos-14, windows-2022 ]
-        # 3.14t is the free-threaded build. No 3.13t: NumPy ships no cp313t
-        # wheels from 2.5 on, so it would compile NumPy from source.
-        python-version: [ "3.11", "3.12", "3.13", "3.14", "3.14t" ]
+        # 3.14t and 3.15t are the free-threaded builds. No 3.13t: NumPy ships no
+        # cp313t wheels from 2.5 on, so it would compile NumPy from source.
+        python-version: [ "3.11", "3.12", "3.13", "3.14", "3.14t", "3.15", "3.15t" ]
 
     env:
       PYTHON_VERSION: ${{ matrix.python-version }}
@@ -60,9 +60,9 @@ jobs:
           platforms: all
 
       - name: Build wheels
-        uses: pypa/cibuildwheel@v3.3.0
+        uses: pypa/cibuildwheel@v4.2.1
         env:
-          # cp314t needs no CIBW_ENABLE; cibuildwheel gates only cp313t.
+          # cibuildwheel >= 4.2 builds cp314/cp315 (incl. t) by default; no CIBW_ENABLE.
           CIBW_BUILD: "cp3${{ env.PYTHON_SUBVERSION }}-*"
           CIBW_SKIP: "*-win32 *-manylinux_i686 *-musllinux*"
           CIBW_ARCHS_MACOS: "x86_64 universal2"