Unable to use templated class as base class in C++ inheritance

[haoyu]

There is no way to declare a C++ class that inherits from a template class yet. The following code will raise syntax error:

from libcpp.vector import vector
cdef extern from "foo.h":
    cdef cppclass VectOfInt(vector[int])

The attached patch could fix this. But the patch needs review and cleanup since I'm unsure about the correctness.