Added in r80244. This new option willl be available in the next 3.2 release. Until 3.2 is released, what you can do is catch the errors
and re-raise the ones that are not due to dangling symlinks:

(not tested)

>>> from shutil import copytree, Error
>>> try:
...     shutil.copytree('../test', '../test2')
... except Error, e:
...     for src, dst, error in e.args[0]:
...         if not os.path.islink(src):
...             raise
...         else:
...             linkto = os.readlink(srcname)
...             if os.path.exists(linkto):
...                 raise
...         # dangling symlink found.. ignoring..
...