Small fixes for roslaunch-check on Python 3.

[mikepurvis]

Stuff that came up in the course of our migration work.

[mikepurvis]

I believe this is the intended use given that the junitxml functions explicitly return encoded strings/bytes, see:

https://github.com/ros/ros/blob/9f2051dd809352fa11bab4671f47d95129125a2e/tools/rosunit/src/rosunit/junitxml.py#L495

And the xml output elsewhere has tests which confirm that it returns bytes, eg: ros/ros#212

[mikepurvis]

Ugh, the new logic only works for Python 3's minidom, as the 2.7 version doesn't supply an implementation of __contains__:

  File "/tmp/ws/src/ros_comm/tools/roslaunch/src/roslaunch/depends.py", line 99, in _check_ifunless
    if 'if' in tag.attributes:
  File "/usr/lib/python2.7/xml/dom/minidom.py", line 522, in __getitem__
    return self._attrs[attname_or_tuple]

I don't see how to get out of making this a versioned check, possibly even as simple as hasattr(dict, 'has_key'). Will update.

[dirk-thomas]

Thanks for the fix.