EXCEPTION_ACCESS_VIOLATION when importing `math`

I simply ran the example, and got an `EXCEPTION_ACCESS_VIOLATION`

```
> using PyCall
> @pyimport math
Please submit a bug report with steps to reproduce this fault, and any error messages that follow (in their entirety). Thanks.
Exception: EXCEPTION_ACCESS_VIOLATION at 0x574c88f -- PyObject_IsInstance at (unknown line)
PyObject_IsInstance at C:\tools\cygwin\python27.DLL (unknown line)
pyisinstance at C:\Users\Joseph\.julia\v0.3\PyCall\src\PyCall.jl:73
pyptr_query at C:\Users\Joseph\.julia\v0.3\PyCall\src\conversions.jl:152
pytype_query at C:\Users\Joseph\.julia\v0.3\PyCall\src\conversions.jl:767
convert at C:\Users\Joseph\.julia\v0.3\PyCall\src\conversions.jl:817
jl_apply_generic at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
pywrap at C:\Users\Joseph\.julia\v0.3\PyCall\src\PyCall.jl:325
pywrap at C:\Users\Joseph\.julia\v0.3\PyCall\src\PyCall.jl:318
jl_apply_generic at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_expr at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_thunk_with at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_thunk_with at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_interpret_toplevel_expr_in at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_eval_with_compiler_p at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
jl_f_top_eval at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
eval_user_input at REPL.jl:53
jlcall_eval_user_input_1027 at (unknown line)
jl_apply_generic at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
anonymous at task.jl:95
jl_handle_stack_switch at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
julia_trampoline at C:\Users\Joseph\AppData\Local\Julia-0.3.5\bin\libjulia.dll (unknown line)
unknown function (ip: 4202784)
unknown function (ip: 4199370)
unknown function (ip: 4199672)
BaseThreadInitThunk at C:\WINDOWS\system32\KERNEL32.DLL (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)
RtlUserThreadStart at C:\WINDOWS\SYSTEM32\ntdll.dll (unknown line)
```

Reproduce the Windows failure with `using PyCall` and `@pyimport math`, then inspect `src/PyCall.jl` and `src/conversions.jl` around `pyisinstance`, `pyptr_query`, and `pytype_query`. Done means importing `math` no longer raises `EXCEPTION_ACCESS_VIOLATION` and the reported example works.

Vom Indexierungsmodell aus dem Issue-Text verfasst.