Remove private _PyDict_GetItemStringWithError() function of the
public C API: the new PyDict_GetItemStringRef() can be used instead.
* Move private _PyDict_GetItemStringWithError() to the internal C API.
* _testcapi get_code_extra_index() uses PyDict_GetItemStringRef().
Avoid using private functions in _testcapi which tests the public C
API.
These functions are broken by design because they discard any exceptions raised
inside, including MemoryError and KeyboardInterrupt. They should not be
used in new code.
Serhiy Storchaka
Victor Stinner
Dong-hee Na
Petr Viktorin