meson-python: _get_macosx_platform_tag: ValueError: not enough values to unpack (expected 2, got 1)

[mkoeppe]

Both ARM (https://github.com/passagemath/passagemath/actions/runs/18730263611/job/53428828938#step:13:14638) and x86_64 (https://github.com/passagemath/passagemath/actions/runs/18730263611/job/53428828947#step:13:14405) are affected

[mkoeppe]

• BUG: allow $MACOSX_DEPLOYMENT_TARGET to be set to major version only mesonbuild/meson-python#762 (not yet in a release)