Enable uv as an alternative to pip installation with --install-types

[cthoyt]

Currently, if you want to use mypy with mypy --install-types, it requires pip to be installed.

In my case, I am using tox with the tox-uv backend that creates all virtual environments without pip, since uv pip install is a drop-in replacement for pip install. I'm calling mypy in these environments, so I would like to have the possibility to specify the installer used.

The main code that would need to be changed is the following:

mypy/mypy/main.py

Line 1576 in fa01a07

cmd = [options.python_executable, "-m", "pip", "install"] + packages

where it might be nice to have the ability to pass in to the enclosing function install_types() an additional keyword called "installer" (or synonym) that would enable changing it to something like:

if installer == "pip":
    assert options.python_executable, "Python executable required to install types with pip"
    cmd = [options.python_executable, "-m", "pip", "install"] + packages
elif installer == "uv":
    import shutil

    # there are two possibilities here, since there might be a globally
    # installed uv that is not managed by the current python environment
    if shutil.which("uv"):  
        cmd = ["uv", "pip", "install"] + packages
    else:
        assert options.python_executable, "Python executable required to install types with uv"
        cmd = [options.python_executable, "-m", "uv", "pip", "install"] + packages
else:
    raise ValueError(f"Unknown installer: {installer}")

This is just a quick, self-contained mockup - there are other patterns that could be used to make this a bit more extensible in case there are other installers desired.

I've got a working demo in master...cthoyt:mypy:uv-installer that I could pull request, but it's not clear to me what is the best way to test this within MyPy's CI