Bug report

Bug description:

import ntpath
print(ntpath.join('c:', 'x', 'y'))

I would expect the following code to print 'c:\\x\\y', yet it prints 'c:x\\y', Would you expect a PR that changes the current behaviour?

CPython versions tested on:

3.10, 3.11, 3.12, 3.13, CPython main branch

Operating systems tested on:

Linux, Windows