[Cython] Cython bugfix release

[Robert Bradshaw]

I just pushed another bugfix release for the 0.20.x line, available on
github, cython.org, or and pypi.

== Features added ==

* Some optimisations for set/frozenset instantiation.
* Support for C++ unordered_set and unordered_map.

== Bugs fixed ==

* Access to attributes of optimised builtin methods (e.g.
[].append.__name__) could fail to compile.
* Memory leak when extension subtypes add a memory view as attribute
to those of the parent type without having Python object attributes or
a user provided dealloc method.
* Compiler crash on readonly properties in "binding" mode.
* Auto-encoding with c_string_encoding=ascii failed in Py3.3.
* Crash when subtyping freelist enabled Cython extension types with
Python classes that use __slots__.
* Freelist usage is restricted to CPython to avoid problems with other
Python implementations.
* Memory leak in memory views when copying overlapping, contiguous slices.
* Format checking when requesting non-contiguous buffers from
cython.array objects was disabled in Py3.
* C++ destructor calls in extension types could fail to compile in clang.
* Buffer format validation failed for sequences of strings in structs.
* Docstrings on extension type attributes in .pxd files were rejected.

== Contributors ==

Andreas van Cranenburgh
Ian Bell
Lars Buitinck
Martin Quarda
Mikhail Korobov
Robert Bradshaw
Stefan Behnel