In docs, it mentions that the function returns true if the base is a 'suburi' of test, but in reality, if is_suburi(base, test) then is_suburi(test, base) which is wrong in most cases (other than where base ~= test, meaning they are the same location)
I am unsure which versions of python are affected, but i suspect most of them (although we probably shouldn't backport this fix?)

Cheryl, do you have time to review this?

Yair, is this really a separate bug from that reported in #42766?  In other words, are there separate and distinct failing test cases?  If not, this should be closed as a duplicate.

This issue was discovered when i was writing unit tests for #42766, since #42766 was fixed when i discovered it, i realized i might want a new issue, i don't mind moving this to be a comment, but there *are* 2 distinct test cases that fail:

a test case that would fail here:
is_suburi('/foo/bar', '/bar/foo') == True
which is different than;
is_suburi('/foo/bar', '/foobar') == True