wurlitzer tries to __enter__ multiple times

[impact27]

I am not sure exactly how I manage to get this error, but here is the traceback I get:

Error in callback <function sys_pipes_forever at 0x11a0d49d8> (for pre_execute):

Traceback (most recent call last):

  File "/usr/local/lib/python3.7/site-packages/wurlitzer.py", line 320, in sys_pipes_forever
    _mighty_wurlitzer.__enter__()

  File "/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 110, in __enter__
    del self.args, self.kwds, self.func

AttributeError: args

Looking at the sys_pipes_forever function, it looks like __enter__ is called even if wurlitzer already called __enter__. As pointed out in the comment above line 110:

        # do not keep args and kwds alive unnecessarily
        # they are only needed for recreation, which is not possible anymore

This means that __enter__ can not be called more than once.

Maybe this could be fixed by changing the indentation of line 320?