fix(pip3): prefer python when available

[SteveLauC]

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

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Closes #470, cc @soredake

Issue

On Windows, when you install python through winget, an actual python binary will be installed as well as a python3 shim, the shim is executable but will launch the Microsoft App store instead of a Python shell. Before this PR, topgrade called python3 under the hood, which is a shim under this case.

Solution

We check both python and python3 through .and_then(check_is_python_2_or_shim), if python is available and is a valid Python 3, we prefer to use it. This is fine as on most systems, python is a link to the default python3.

[soredake]

I will test this if there is a build of topgrade from this pr.

[SteveLauC]

I will test this if there is a build of topgrade from this pr.

Will set up a rust dev environment on my Win VM and upload a build in my fork repo: https://github.com/SteveLauC/topgrade

[SteveLauC]

I will test this if there is a build of topgrade from this pr.

Will set up a rust dev environment on my Win VM and upload a build in my folk repo: https://github.com/SteveLauC/topgrade

There you go: https://github.com/SteveLauC/topgrade/releases/tag/fix-pip3

[soredake]

Works fine now, thanks!