Add environ and environb

[abadger]

Add six.environ and six.environb.

[abadger]

Okay, I've fixed compatibility with Python-2.5 (where we have to use DictMixin because MutableMapping is unavailable) and Python-3.0 and Python-3.1 where we do not have os.environb.

I didn't choose to fix the latter by implementing os.environb. Rather, I chose to fix them by implementing os.supports_bytes_environ. os.supports_bytes_environ is a boolean that defines whether python has an environb on this platform. Currently in the stdlib version, it is False on Windows (os.name != 'nt') and True otherwise. portable software relying on os.environb should be using that to be sure that environb exists on their platform.

The fix extends that concept slightly so that on Python-3.0 and Python-3.1 six.supports_bytes_environ is False, marking it as unavailable no matter the operating system when running on either of those two Python versions.

Note that os.environb could be implemented, similar to how _TextEnviron was implemented, but I wasn't sure if that was desired or even useful so I held off on taking that leap.