use shutil.which() to detect the active python

[dimbleby]

Resolves: #7772

#7075 (comment) says that subprocess.call("python") on windows doesn't give the same answer as calling python from the command line does (in particular, since dropping shell=True on the subprocess call).

dropping shell=True still seems to me to be highly desirable, so this MR uses shutil.which("python") to figure out which python would have been found by typing python directly.

I'm not working on windows so can't readily verify that this works and it's not clear to me how to write a test script either. Such things certainly seem desirable, but it might need someone else to pick up the baton...

[dimbleby]

second commit probably fixes #7772

[xyxel]

shutil.which() works fine. However current fix doesn't work, because
python_path = self._full_python_path(python)
in activate method should be updated too.
Maybe it's better to add shutil.which to _full_python_path method instead of using it outside?

[xyxel]

Now it works. Thank you!

[github-actions]

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