Relax py::pickle() get/set type check

Fixes #1061.

`T` and `const T &` are compatible types.
diff --git a/tests/test_pickling.cpp b/tests/test_pickling.cpp
index 821462a..9dc63bd 100644
--- a/tests/test_pickling.cpp
+++ b/tests/test_pickling.cpp
@@ -115,7 +115,7 @@
             [](py::object self) {
                 return py::make_tuple(self.attr("value"), self.attr("extra"), self.attr("__dict__"));
             },
-            [](py::tuple t) {
+            [](const py::tuple &t) {
                 if (t.size() != 3)
                     throw std::runtime_error("Invalid state!");