not a Python function

sorry if i'm being dumb about this, but functions with multiple input arguments don't seem to be recognized as functions:

```
julia> using PyCall

julia> @pyimport scipy.optimize as so

julia> so.curve_fit((x,m,b)->m*x+b, 1:10, 2:2:20)
ERROR: PyError (:PyObject_Call)
TypeError(' is not a Python function',)
File "/home/arthurb/.julia/v0.4/Conda/deps/usr/lib/python2.7/site-packages/scipy/optimize/minpack.py", line 604, in curve_fit
args, varargs, varkw, defaults = _getargspec(f)
File "/home/arthurb/.julia/v0.4/Conda/deps/usr/lib/python2.7/site-packages/scipy/_lib/_util.py", line 289, in getargspec_no_self
argspec = inspect.getargspec(func)
File "/home/arthurb/.julia/v0.4/Conda/deps/usr/lib/python2.7/inspect.py", line 816, in getargspec
raise TypeError('{!r} is not a Python function'.format(func))

[inlined code] from /home/arthurb/.julia/v0.4/PyCall/src/exception.jl:81
in pycall at /home/arthurb/.julia/v0.4/PyCall/src/PyCall.jl:356
in call at /home/arthurb/.julia/v0.4/PyCall/src/PyCall.jl:372

julia> so.newton(x -> cos(x) - x, 1)
0.7390851332151607
```