Add C API for importing an attribute from a module

[serhiy-storchaka]

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.

[kumaraditya303]

Since this will be private API, how about using statically allocated string identifiers to avoid memory allocations?

[serhiy-storchaka]

The PR adds also function _PyImport_GetModuleAttr() which takes PyObject* arguments. You can use it with statically allocated string identifiers.