run_doctests doesn't reset display manager appropriately

[roed]

sage: run_doctests(sage.rings.ring)
...
sage: True
True
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-819d8d12aa16> in <module>()
----> 1 True

/home/roed/sagedev/local/lib/python2.7/site-packages/IPython/core/displayhook.pyc in __call__(self, result)
    244             self.start_displayhook()
    245             self.write_output_prompt()
--> 246             format_dict, md_dict = self.compute_format_data(result)
    247             self.update_user_ns(result)
    248             self.fill_exec_result(result)

/home/roed/sagedev/local/lib/python2.7/site-packages/IPython/core/displayhook.pyc in compute_format_data(self, result)
    148 
    149         """
--> 150         return self.shell.display_formatter.format(result)
    151 
    152     # This can be set to True by the write_output_prompt method in a subclass

/home/roed/sagedev/local/lib/python2.7/site-packages/sage/repl/display/formatter.pyc in format(self, obj, include, exclude)
    200         """
    201         # First, use Sage rich output if there is any
--> 202         sage_format, sage_metadata = self.dm.displayhook(obj)
    203         assert PLAIN_TEXT in sage_format, 'plain text is always present'
    204         if not set(sage_format.keys()).issubset(self.default_mime()):

TypeError: 'NoneType' object is not iterable

[roed]

The problem is that init_sage in sage.doctest.forker makes changes that aren't reset in the finally block of run_doctests.