Fix python path resolution on linux

[Gitznik]

• I have added an entry to docs/changelog.md

Summary of changes

Only use the logic introduced in #1168 to fix a path resolution issue on windows when pipx is running on a Windows machine.
This fixes #1195

Test plan

When no venv for python3.9 exists, but python is on the PATH (in my case via pyenv):

pipx run --python=python3.9 pycowsay moo

Before this change leads to the error described in #1195 , and works after this change.

Tested by running

pipx run --python=python3.9 pycowsay moo

[robinbowes]

Would love to see this merged & released!

[robinbowes]

LGTM

[gaborbernat]

Test please and merge conflict.

[Gitznik]

Test please and merge conflict.

I added some tests. I could not come up with a good way of reproducing this exact issue in a test suite without requiring multiple python versions to be installed on the runner, which I think would not be great for local execution. Let me know if you have other ideas @gaborbernat.

BTW we could set os.path.realpath to fail if it can't resolve the path (at the moment it does not fail, but instead creates the invalid path on unix based systems), but I was unsure if that would break a lot of things on windows.

[ewjoachim]

@gaborbernat thanks for merging ! In this message, @Gitznik mentionned that they were expecting to change one line, but I think you merged before they had an opportunity to do that (well, the tool was broken, so it's a good decision whatsoever).

Do we want to make a follow-up PR with the suggested simplification, or drop it altogether?