dogwrap python3 string changes

[martin308]

Wrapping a command with python3 is broken due to string changes in python3. This decodes the string using the _fwd_out encoding.

Another option is to just use the utf-8 decoded line below for both.

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/dbt/src/datadog/datadog/dogshell/wrap.py", line 68, in run
    self._fwd_out.write(line)
TypeError: write() argument must be str, not bytes

[agamdua]

Would be great if this patch is accepted and a release is made, many distros include >Python3.5 by default, and installing python2 just to use dogwrap for a cron job wouldn't make sense.

[yannmh]

The changes look good. Thanks @martin308 !