gh-128911: Add PyImport_ImportModuleAttr() function (#128912)

Add PyImport_ImportModuleAttr() and
PyImport_ImportModuleAttrString() functions.

* Add unit tests.
* Replace _PyImport_GetModuleAttr()
  with PyImport_ImportModuleAttr().
* Replace _PyImport_GetModuleAttrString()
  with PyImport_ImportModuleAttrString().
* Remove "pycore_import.h" includes, no longer needed.