New version of my patch, which looks much better. Summary:

  Issue #8603: Create os.environb and os.getenvb() on POSIX system.
  os.unsetenv() encodes str argument using file system encoding and
  surrogateescape error handler (instead of utf8/strict), and accept bytes.

Changes with my previous patch:

 - update os module documentation
 - os.getenv() only accepts str, os.getenvb() only accepts bytes (to avoid mojibake)
 - fix test_environb() of test_os for ASCII locale
 - fix os.putenv() if key is an unicode string: use the string encoded to bytes as key for posix_putenv_garbage
 - _Envion.__setitem__() encodes the key and value before calling putenv()
 - _Environ.__delitem__() encodes the key before calling unsetenv()
 - create a temporary function to create os.environ (to use temporary variables like encode, decode, keymap, data)
 - annotation types in os.getenv() and os.getenvb()
 - remove fsencode()/fsdecode() from the patch and don't touch os._execvpe() (will be fixed in other issues)
 - putenv() uses PyBytes_GET_SIZE() instead of strlen()