Mac: Error pop-up isn't shown on Python 3

[fzumstein]

Reason is that in Python 3, buffering of stderr is the default while in Python 2 it is not: http://python.6.x6.nabble.com/Buffering-of-sys-stdout-and-sys-stderr-in-python3-and-documentation-td1411111.html

Can be fixed with running python -u.

[fzumstein]

Potential cotcha on Mac: If an import fails before xlwings is imported, the error will not be shown as pop-up and the StatusBar will not be reset.