My ~/.ipython profile causes test_backend_nbagg to fail (but is
otherwise valid for my own use). To fix that, point the ipython
directory to a temporary directory when running that test (per
https://ipython.readthedocs.io/en/stable/config/intro.html#the-ipython-directory).

While we're at it, also create the temporary output file in that
directory -- that's quite likely to be necessary for the test to
actually work on Windows, anyways (due to vagaries with writing from
multiple processes to a NamedTemporaryFile).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way