Add C API for importing an attribute from a module

It is common (more than 30 cases) in the C code to access a function or a variable from other module. For example:

    functools = PyImport_ImportModule("functools");
    if (!functools)
        goto error;
    st->partial = PyObject_GetAttrString(functools, "partial");
    Py_CLEAR(functools);

I propose to add a private helper function which combines PyImport_ImportModule() and PyObject_GetAttrString().

    st->partial = _PyImport_GetModuleAttrString("functools", "partial");

It will save 4-6 lines of code and a variable on every use.