fix ros_location_find with Python 3

[seanyen]

Since Python 3, the subprocess.Popen has a text/non-text mode which decides the data type to use for communicate.

Source: https://docs.python.org/3.8/library/subprocess.html#subprocess.Popen.communicate

If streams were opened in text mode, input must be a string. Otherwise, it must be bytes.

And without extra conversion, the rosfindpath.py will fail with the following trackback:

Traceback (most recent call last):
  File "c:\opt\ros\noetic\x64\bin\\rosfindpath.py", line 82, in <module>
    sys.exit(findpathmain(sys.argv[1:]))
  File "c:\opt\ros\noetic\x64\bin\\rosfindpath.py", line 74, in findpathmain
    rosdir = os.path.normpath(os.path.sep.join([package_dir, reldir]))
TypeError: sequence item 0: expected str instance, bytes found

This pull request is try to use decode() to convert it back to str to make it type correct.

This fix was motivated by the issue reported here: ms-iot/ROSOnWindows#248

[seanyen]

cc @dirk-thomas @sloretz This is ready for review and merge. Thanks!

[dirk-thomas]

Thanks for the fix.