parse_package_xml.py: Always write output file as UTF-8 for python3.

[de-vri-es]

In python3, reading or writing text files by use a default encoding depending on your locale. That means that handling of package.xml file containing UTF-8 without a UTF-8 locale doesn't work. See below for an example error.

This PR makes sure UTF-8 is used as encoding with python3 regardless of your locale. Note that this is a essentially the same issue as fixed in ros-infrastructure/catkin_pkg#143.

Traceback (most recent call last):
  File "/opt/ros/kinetic/share/catkin/cmake/parse_package_xml.py", line 107, in <module>
    main()
  File "/opt/ros/kinetic/share/catkin/cmake/parse_package_xml.py", line 103, in main
    ofile.write('\n'.join(lines))
UnicodeEncodeError: 'ascii' codec can't encode character '\u0160' in position 107: ordinal not in range(128)

[dirk-thomas]

Thanks.