Simplify tests

Remove testing of setting the env vars that tempfile.gettempdir() uses.
Why?

- The existing tests failed on my system
- In general, tempfile.tempdir is system-dependent and hence not easy to
  test.
- The test would essentially test that tempfile does what it says it
  does, and this module is not in the business of testing tempfile.
Add Changelog.  Bump version.