test suite uses deprecated mktemp

According to the Python docs, [mktemp](https://docs.python.org/3/library/tempfile.html#tempfile.mktemp) is deprecated since Python 2.3, but it's still in use in test_find_packages. Let's replace that with something else.

Originally reported at https://huntr.com/bounties/d7dae84c-e779-4895-a3a1-af9e3bed0f5e.