Use shutil when looking for executables on the host system

[mkrasnitski]

Description

The implementation of the which function only checks for the executable flag on paths, which means it can return a directory (at least on POSIX), causing weird permission errors down the line. It should instead defer to shutil.which; that function contains a platform-agnostic implementation that should hopefully match the behavior of the GNU which utility.

Checklist

• [x] My code follows the code style of this project.
• [x] My change includes a change to the documentation, if required.
• [x] If my change adds new code, adequate tests have been added.
• [x] I have read and agree to the CONTRIBUTING document.

[github-actions[bot]]

🤖 Coverage update for f053502116ef4eafc4ab9dd561d1f6e4a9c2108c 🟢

	Old	New
Commit	ba70548a3ee24355c1c6daca0aa302d326dc2335	f053502116ef4eafc4ab9dd561d1f6e4a9c2108c
Score	71.2773%	71.2773% (0)

[github-actions[bot]]

🤖 Coverage update for a00d9f15385eaf0bcfebc2442e352a4391d3c08e 🟢

	Old	New
Commit	ba70548a3ee24355c1c6daca0aa302d326dc2335	a00d9f15385eaf0bcfebc2442e352a4391d3c08e
Score	71.2773%	71.2773% (0)

[github-actions[bot]]

🤖 Coverage update for 13ff22c341453c69dfc36b350b1f356bceecd0f6 🟢

	Old	New
Commit	ba70548a3ee24355c1c6daca0aa302d326dc2335	13ff22c341453c69dfc36b350b1f356bceecd0f6
Score	71.2773%	71.2773% (0)