Fix webcollab.py to be more Python3 compatible

Change c3770c88b50 in #1636 introduced a new python source file, `webcollab.py`, containing some print statements, a well-known incompatibility at the source level between Python 2 and Python 3.

Starting an updated fontforge built with Python 3 produces this error:
```
  File "/home/tom/local/share/fontforge/python/webcollab.py", line 39
    print 'webcollab closing...'
                               ^
SyntaxError: invalid syntax
```

This fix merely changes to the Python 3 function call format for print.  It seems to satisfy both Python3 and Python2...