How would this feature be useful?
When one creates a new python package, with dependencies, and then
uploads the new package to test.pypi.org, one would like to test using pipx to install the package from test.pypi.org.
But this does not work if the new package depends on packages on pypi.org. This is because pipx does not support the argument --extra-index-url https://pypi.org/simple/. pip supports this argument, but not pipx.

When testing using pipx to install a newly created, or significantly changed, python package from test.pypi.org.

Describe the solution you'd like
Support argument --extra-index-url https://pypi.org/simple/

Describe alternatives you've considered
Sorry, I have no idea about alternatives.