Installing with --python-version doesn't resolve deps for indicated version

[jaraco]

Description

I recently attempted to install to --target using --python-version. After learning that I also had to supply --only-binary :all:, the command completed, but relevant dependencies dependent on the Python version were missing.

 @ py -m pip install --python-version 3.8 --only-binary :all: -t ~/.local/pip-run pip-run
...
 @ env PYTHONPATH=~/.local/pip-run py -3.8 -m pip-run
Traceback (most recent call last):
  File "/opt/homebrew/Cellar/python@3.8/3.8.19/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/opt/homebrew/Cellar/python@3.8/3.8.19/Frameworks/Python.framework/Versions/3.8/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/jaraco/.local/pip-run/pip-run.py", line 1, in <module>
    from pip_run import run
  File "/Users/jaraco/.local/pip-run/pip_run/__init__.py", line 3, in <module>
    from . import commands, deps, launch, scripts
  File "/Users/jaraco/.local/pip-run/pip_run/commands.py", line 6, in <module>
    from jaraco.context import suppress
  File "/Users/jaraco/.local/pip-run/jaraco/context.py", line 17, in <module>
    from backports import tarfile
ModuleNotFoundError: No module named 'backports'

It seems backports.tarfile wasn't installed, even though it's required on Python 3.8.

Expected behavior

Install with --python-version should resolve dependencies the same as if the command had been run on that Python version.

pip version

24.0

Python version

3.12

OS

Windows/macOS

How to Reproduce

See description

Output

No response

Code of Conduct

• I agree to follow the PSF Code of Conduct.

[ichard26]

This is likely a manifestation of #11664.

[ichard26]

Actually, I'm just going to merge this into #11664.