Python 3 fixes
Fix in test_pv
Fixes in test_custom_viewer
Don’t rely on dictionary order
Skip test_choose_save_session_ioerror in Python 3
Enable all tests in Python 3
Make order of choice elements deterministic
Fix test_choose_save_session_ioerror
Merge pull request #458 from astrofrog/fix-qt-python3

Final round of Python 3 fixes (requires #457 to be merged first)