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

Fixes #1061.

`T` and `const T &` are compatible types.
diff --git a/docs/changelog.rst b/docs/changelog.rst
index b9d630d..b4eecab 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -17,6 +17,9 @@
 * Fixed a regression where the ``py::keep_alive`` policy could not be applied
   to constructors. `#1065 <https://github.com/pybind/pybind11/pull/1065>`_.
 
+* Relax overly strict ``py::picke()`` check for matching get and set types.
+  `#1064 <https://github.com/pybind/pybind11/pull/1064>`_.
+
 v2.2.0 (August 31, 2017)
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