Replace additional use of `which(1)` with `shutil.which()`

[mgorny]

Replace the remaining use of external which(1) tool with shutil.which() from the standard Python library, finally removing the dependency on a third party package.

This is a followup to 1024f4f64ceabd612b4df9a0b9dbe2691b2f5f9d.

[ekalinin]

Hey! Thanks for the patch! Could you, please, add some test?

[mgorny]

What kind of test?

[ekalinin]

Unit test.

[mgorny]

install_activate is already covered by tests. I fail to see what I could test differently there.