Restore support for install via pip.

[jaraco]

Closes #3143

Summary of changes

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[jaraco]

@hroncok Would you mind reviewing this change? Will it satisfy the use case, at least until November?

[hroncok]

This adds a dependency on pip:

$ python setup.py install --prefix ../../fake_prefix
running install
.../venv/lib64/python3.13/site-packages/setuptools/_distutils/cmd.py:90: SetuptoolsDeprecationWarning: setup.py install is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` directly.
        Instead, use pypa/build, pypa/installer or other
        standards-based tools.

        See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
        ********************************************************************************

!!
  self.initialize_options()
.../venv/lib64/python3.13/site-packages/setuptools/_distutils/cmd.py:90: InstallDeprecationWarning: install command is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` and ``install``.
        Instead, use standards-based tools like pip or uv.

        By 2025-Oct-31, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://github.com/pypa/setuptools/issues/917 for details.
        ********************************************************************************

!!
  self.initialize_options()
.../venv/bin/python: No module named pip
Traceback (most recent call last):
  File ".../empty/setup.py", line 3, in <module>
    setup(name='wtf', version='1')
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/__init__.py", line 117, in setup
    return distutils.core.setup(**attrs)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/_distutils/core.py", line 186, in setup
    return run_commands(dist)
  File ".../venv/lib64/python3.13/site-packages/setuptools/_distutils/core.py", line 202, in run_commands
    dist.run_commands()
    ~~~~~~~~~~~~~~~~~^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/_distutils/dist.py", line 1002, in run_commands
    self.run_command(cmd)
    ~~~~~~~~~~~~~~~~^^^^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/dist.py", line 1104, in run_command
    super().run_command(command)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/_distutils/dist.py", line 1021, in run_command
    cmd_obj.run()
    ~~~~~~~~~~~^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/command/install.py", line 110, in run
    self.do_pip_install()
    ~~~~~~~~~~~~~~~~~~~^^
  File ".../venv/lib64/python3.13/site-packages/setuptools/command/install.py", line 153, in do_pip_install
    subprocess.check_call(cmd)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "/usr/lib64/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['.../venv/bin/python', '-m', 'pip', 'install', '.', '--use-pep517', '--prefix', '../../fake_prefix']' returned non-zero exit status 1.

Meanwhile, using --root still works:

$ python setup.py install --root ../.. --prefix /fake_prefix
running install
.../venv/lib64/python3.13/site-packages/setuptools/_distutils/cmd.py:90: SetuptoolsDeprecationWarning: setup.py install is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` directly.
        Instead, use pypa/build, pypa/installer or other
        standards-based tools.

        See https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html for details.
        ********************************************************************************

!!
  self.initialize_options()
.../venv/lib64/python3.13/site-packages/setuptools/_distutils/cmd.py:90: InstallDeprecationWarning: install command is deprecated.
!!

        ********************************************************************************
        Please avoid running ``setup.py`` and ``install``.
        Instead, use standards-based tools like pip or uv.

        By 2025-Oct-31, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://github.com/pypa/setuptools/issues/917 for details.
        ********************************************************************************

!!
  self.initialize_options()
running build
running install_egg_info
running egg_info
writing wtf.egg-info/PKG-INFO
writing dependency_links to wtf.egg-info/dependency_links.txt
writing top-level names to wtf.egg-info/top_level.txt
reading manifest file 'wtf.egg-info/SOURCES.txt'
writing manifest file 'wtf.egg-info/SOURCES.txt'
removing '../../fake_prefix/lib/python3.13/site-packages/wtf-1-py3.13.egg-info' (and everything under it)
Copying wtf.egg-info to ../../fake_prefix/lib/python3.13/site-packages/wtf-1-py3.13.egg-info
running install_scripts

As such, I don't believe this really fixes the problem.

[jaraco]

Indeed, it does add a dependency on pip. I was thinking that would be acceptable, that pip could be an implied dependency for these workflows, but it sounds like that's not the case. I'll revisit in the ticket.