manage stderr more dynamically, to avoid failures like this that can …

[artgoldberg]

…occur when concurrent programs close & reopen stderr

self = <progress.bar.Bar object at 0x7f89ed7b1438>

    def is_tty(self):
>       return self.file.isatty() if self.check_tty else True
E       ValueError: I/O operation on closed file

We have a set of build utilities that won't work with progress without these changes.

Thanks Arthur

[verigak]

I'm afraid this will break code that has overriden file to something else.