python stack frames not initialized (inspect.currentframe() fails)

I was trying to use `PyCall` to import `ggplot` from https://github.com/yhat/ggplot/

The `qplot` function works fine.

```
ggplot.qplot([1,2],[3,4])
```

However, I can never get the `ggplot` function working. Specially, I can never create the `aes` class

```
julia> using PyCall

julia> @pyimport ggplot

julia> ggplot.aes
PyObject

julia> ggplot.aes(x="x",y="y")
ERROR: type: apply: expected Function, got PyObject

julia> pycall(ggplot.aes, PyObject, x="x",y="y")
ERROR: PyError (PyObject_Call)
AttributeError("'NoneType' object has no attribute 'f_locals'",)
File "/Users/Randy/.pyenv/versions/2.7.6/Python.framework/Versions/2.7/lib/python2.7/site-packages/ggplot/components/aes.py", line 68, in __init__
self.__eval_env__ = EvalEnvironment.capture(1)
File "/Users/Randy/.pyenv/versions/2.7.6/Python.framework/Versions/2.7/lib/python2.7/site-packages/patsy/eval.py", line 179, in capture
return cls([frame.f_locals, frame.f_globals],

in pyerr_check at /Users/Randy/.julia/v0.3/PyCall/src/exception.jl:58
in pycall at /Users/Randy/.julia/v0.3/PyCall/src/PyCall.jl:85
```

It seems that `ggplot` is calling `patsy` and checking some stack frame stuff.

---

Further study reveals that the root of the error is `.../patsy/eval.py` line 179

```
frame = inspect.currentframe()
try:
for i in xrange(depth + 1):
if frame is None:
raise ValueError, "call-stack is not that deep!"
frame = frame.f_back
return cls([frame.f_locals, frame.f_globals],
frame.f_code.co_flags & _ALL_FUTURE_FLAGS)
```

I have no idea what happens here...

Reproduce the failure by calling ggplot.aes through PyCall, then inspect patsy/eval.py at line 179 and the related PyCall paths in src/exception.jl and src/PyCall.jl. Determine why inspect.currentframe() has no usable frame in this call path; done means the aes constructor completes without the reported AttributeError.

Scritto dal modello di indicizzazione a partire dal testo della issue.