Prefer item lookup when converting mappings to objects - #1266

Tested this (272b924) on Python 3.11 against main 75e1341. On main, #1246 reproduces for UserDict and sqlite3.Row (a keys column gives "Expected int, got builtin_function_or_method"). With this branch those convert correctly, and so do attrs targets, tagged struct unions, and fields named pop, setdefault, update, clear, copy, fromkeys. pytest tests/unit passes (6347 passed, 324 skipped).

One regression though: anything that has __getitem__ but exposes fields only as attributes no longer converts without from_attributes=True. SQLAlchemy 2.x Row is the common case, since row["id"] raises and the columns are attributes:

import sqlalchemy as sa, msgspec

class U(msgspec.Struct):
    id: int
    name: str

with sa.create_engine("sqlite://").connect() as c:
    row = c.execute(sa.text("select 1 as id, 'x' as name")).first()
    msgspec.convert(row, U)

main: U(id=1, name='x'). This branch: ValidationError: Object missing required field `id` (SQLAlchemy 2.1.4). Same for a plain class with __getitem__ plus attributes. The PR also drops the (GetAttrOrItemObj, False, False) row from the test table in test_convert.py, which is the test that covered this case.

Trying the item first and falling back to the attribute keeps both working:

static PyObject *
getitem_then_getattr(PyObject *obj, PyObject *key) {
    PyObject *out = PyObject_GetItem(obj, key);
    if (out == NULL) {
        PyErr_Clear();
        out = PyObject_GetAttr(obj, key);
    }
    return out;
}

with getter = getitem_then_getattr; in the from_attributes=False branch. With that, the unit suite still passes (6347 passed), including your new tests, the #1246 cases above convert, and the SQLAlchemy row converts again. The one thing it doesn't cover is a mapping that lacks a keys key while the target has an optional keys field: that still picks up the bound method, but that is what main does today. Maybe worth a test with an attribute-only __getitem__ object either way.