The use of `matrix.NIGHTLY`, `matrix.EXTRACT`, etc. makes CI matrix
configuration difficult. Instead, let the shell script for the neovim
installation step take care of OS specializations.

Problem: macos-latest now points to M1 runners (macos-14-arm64) which do not
support python 3.7 - 3.9.

Solution: For these python versions, use the macos-12 (x86_64) runner.

    running bdist_wheel
    /usr/lib/python3/dist-packages/setuptools/_distutils/cmd.py:111: SetuptoolsDeprecationWarning: bdist_wheel.universal is deprecated
    !!

            ********************************************************************************
            With Python 2.7 end-of-life, support for building universal wheels
            (i.e., wheels that support both Python 2 and Python 3)
            is being obviated.
            Please discontinue using this option, or if you still need it,
            file an issue with pypa/setuptools describing your use case.

            By 2025-Aug-30, you need to update your project and remove deprecated calls
            or your builds will no longer be supported.
            ********************************************************************************

    !!

fix(ci): macOS CI failing with python{3.7, 3.8, 3.9}

Fix conditional `install_requires` in `setup.py` to use environment
markers instead of inline conditions.  The latter do not work correctly
with universal wheels -- e.g. a wheel made on Python 3.12 would not have
a dependency on `typing-extensions` at all, even if it were installed
on Python 3.11 or older, and the other way around.

- a2adeec build: fix conditional install_requires #581
- 31550a7 fix: remove deprecated universal wheels setting
- a6175a4 fix(ci): macOS CI failing with python{3.7, 3.8, 3.9}
- 648b0c7 fix(ci): do not use matrix variables for OS-specific commands