Fix py::cast from pytype rvalue to pytype (#3949)

* Fix py::cast from pytype rvalue to pytype

Previously, py::cast blindly assumed that the destination type was a C++
type rather than a python type when the source type was an rvalue.

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tests/test_copy_move.py

@@ -123,3 +123,10 @@ def test_move_fallback():
     assert m1.value == 1
     m2 = m.get_moveissue2(2)
     assert m2.value == 2
+
+
+def test_pytype_rvalue_cast():
+    """Make sure that cast from pytype rvalue to other pytype works"""
+
+    value = m.get_pytype_rvalue_castissue(1.0)
+    assert value == 1