C API: Add PyObject_GetOptionalAttr() function

[serhiy-storchaka]

It is a new name of former _PyObject_LookupAttr() added in #76752.

int PyObject_GetOptionalAttr(PyObject *obj, PyObject *attr_name, PyObject **result);

Discussion about making it public and naming: https://discuss.python.org/t/make-pyobject-lookupattr-public/29104.

See also other new functions with similar interface in #106307 and #106004.

Linked PRs

• gh-106521: Add PyObject_GetOptionalAttr() function #106522
• gh-106521: Remove _PyObject_LookupAttr() function #106642

[vstinner]

I added PyObject_GetOptionalAttr() and PyObject_GetOptionalAttrString() to pythoncapi-compat: python/pythoncapi-compat@157d6d4

[carljm]

PRs are merged, this looks done.