Before passing objects to _chanselect for send, pyselect increfs them, as just
send does, to indicate that one object reference is passed to channel buffer.
On exit, since only one case is actually executed by select, pyselect needs to
decref incref'ed object from not executed cases.

Pyselect already implements the latter cleanup, but currently the cleanup is
executed only if control flow reaches _chanselect at all. Which is a bug, since
pyselect can panic or raise an exception just in the middle of preparation phase.

-> Fix it by associating the finally-decref cleanup with whole
prepare+_chanselect code.

Without the fix, the second part of added test (abnormal exit) fails e.g. like:

        @mark.skipif(not hasattr(sys, 'getrefcount'),   # skipped e.g. on PyPy
                     reason="needs sys.getrefcount")
        def test_select_refleak():
            ch1 = chan()
            ch2 = chan()
            obj1 = object()
            obj2 = object()
            tx1 = (ch1.send, obj1)
            tx2 = (ch2.send, obj2)

            # normal exit
            gc.collect()
            nref1 = sys.getrefcount(obj1)
            nref2 = sys.getrefcount(obj2)
            _, _rx = select(
                tx1,        # 0
                tx2,        # 1
                default,    # 2
            )
            assert (_, _rx) == (2, None)
            gc.collect()
            assert sys.getrefcount(obj1) == nref1
            gc.collect()
            assert sys.getrefcount(obj1) == nref2

            # abnormal exit
            with raises(AttributeError) as exc:
                select(
                    tx1,        # 0
                    tx2,        # 1
                    'zzz',      # 2 causes pyselect to panic
                )
            assert exc.value.args == ("'str' object has no attribute '__self__'",)
            gc.collect()
    >       assert sys.getrefcount(obj1) == nref1
    E       assert 4 == 3
    E         -4
    E         +3

    golang/golang_test.py:690: AssertionError

The bug was introduced in 3b241983 (Port/move channels to C/C++/Pyx).