The changes following r67300 introduced the .mode and .name attributes on all 
the file-like objects returned by the open() function.

This also changed the mode returned by a file opened in binary mode: it was 
"rb", now it is "r". The fact that the mode does not "round-trip" (i.e: open(f, 
mode).mode != mode) was considered not important.

But now it is difficult to see if some opened file was opened in text or binary 
mode; in test_gzip.py, a test had to be changed, and now it does not test 
anything at all: the intent of the test is just to verify that a zip file is 
always opened in binary mode.

Benjamin suggested to change the mode returned by FileIO objects, so that they 
always contain a 'b'. They also accept an extra 'b' on creation: it is just 
ignored.

Now, for a file opened in text mode:
>>> f = open('filename')
>>> assert f.mode            == 'r'
>>> assert f.buffer.mode     == 'rb'
>>> assert f.buffer.raw.mode == 'rb'

The mode attribute is now always consistent with the one passed to the open() 
function. This also avoid gratuitous breakage with python2.x.

Patch attached. All tests pass.