> @loewis: So do you agree to add os.environb and os.getenvb()?

I agree with the patch (-2) in principle. I think the error handling
needs to improve:

py> os.getenvb('PATH')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/martin/work/3k/Lib/os.py", line 484, in getenvb
    return environb.get(key, default)
  File "/home/martin/work/3k/Lib/_abcoll.py", line 357, in get
    return self[key]
  File "/home/martin/work/3k/Lib/os.py", line 400, in __getitem__
    value = self.data[self.encodekey(key)]
TypeError: string argument without an encoding

which then leads to the natural, but incorrect

py> os.getenvb('PATH', encoding='ascii')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: getenvb() got an unexpected keyword argument 'encoding'

The first error should remain TypeError, but say something like
"unsupported type string", or "bytes expected".

I notice an incompatible change: posix.environ has now a different
element type. This is probably fine.

There is a couple of white-space only changes in the patch; it would be
good if you could reduce them.