[PATCH] py3: handle os.environ.get() case in module loader

[Jun Wu]

util.environ looks better. ui.environ has issues for places without the ui
object.

I suggested change the transformer in the previous py3 thread because I felt
that was better than changing individual files manually.

Excerpts from Siddharth Agarwal's message of 2016-08-04 20:26:54 -0700:
> I agree with Greg -- this makes things more complicated than necessary. 
> We should just have a helper (e.g. util.environ) that gets assigned to 
> os.environ on py2 and os.environb on py3. (And with possibly different 
> behavior on Windows, similar to filenames.)
> 
> - Siddharth