getiter: use tp_iter for non python types

[undingen]

                           ea3daebf08d007db3e:  7075b3803b12f58f8f:
      django_template3.py             3.3s (4)             3.3s (4)  -0.3%
            pyxl_bench.py             2.8s (4)             2.8s (4)  -1.3%
sqlalchemy_imperative2.py             3.2s (4)             3.2s (4)  -2.3%
                  geomean                 3.1s                 3.1s  -1.3%

[kmod]

I think sharing the code between the capi (tp_iter) and c++ (__iter__ attribute) versions is fine since none of these functions should throw any exceptions, but maybe we could add noexcept to these functions?

[undingen]

I added the noexcept specifiers.

[kmod]

thanks :) I'm trying to think of a way to have callers also specify the exception-style they expect their callee to have, and then hopefully get the type system to enforce a lot of this so we don't have to think about it any more.