pudb isn't compatible w/ mercurial.demandimport

Mercurial has a magical import wrapper called demandimport.

``` py
import mercurial
import mercurial.demandimport
mercurial.demandimport.enable()
import foo
```

if foo does not exist, it will not throw an exception at this point.
Instead, it will throw its exception when you do:

``` py
foo.bar
```

I realized this when using **py3** (not **IronPython**) w/ mercurial.demandimport and **pudb3**:

``` py
>>> ctypes.util
Traceback (most recent call last):
File "/home/timeless/hg/py3/lib/python3.5/site-packages/pudb/debugger.py", line 1415, in
cmdline_get_namespace
from pudb.shell import SetPropagatingDict
File "/home/timeless/hg/crew/mercurial/demandimport.py", line 143, in _demandimport
return _hgextimport(_import, name, globals, locals, fromlist, level)
File "/home/timeless/hg/crew/mercurial/demandimport.py", line 53, in _hgextimport
return importfunc(name, globals, *args, **kwargs)
File "/home/timeless/hg/py3/lib/python3.5/site-packages/pudb/shell.py", line 183, in
if HAVE_IPYTHON and hasattr(IPython, 'Shell'):
File "/home/timeless/hg/crew/mercurial/demandimport.py", line 132, in __getattribute__
self._load()
File "/home/timeless/hg/crew/mercurial/demandimport.py", line 98, in _load
mod = _hgextimport(_import, head, globals, locals, None, level)
File "/home/timeless/hg/crew/mercurial/demandimport.py", line 53, in _hgextimport
return importfunc(name, globals, *args, **kwargs)
ImportError: No module named 'IPython'
```

The fix for this sort of thing is to access a property of each module inside the try block that intends to import the module. I'll try to write a PR for this, but first I'll need to find out what properties IPython has (and probably try installing it...