py-cython: updated to 0.29.20

0.29.20:

Bugs fixed

* Nested try-except statements with multiple ``return`` statements could crash
  due to incorrect deletion of the ``except as`` target variable.
* The ``@classmethod`` decorator no longer rejects unknown input from other decorators.
* Fused types could leak into unrelated usages.
* Now uses ``Py_SET_SIZE()`` and ``Py_SET_REFCNT()`` in Py3.9+ to avoid low-level
  write access to these object fields.
* The built-in ``abs()`` function could lead to undefined behaviour when used on
  the negative-most value of a signed C integer type.
* Usages of ``sizeof()`` and ``typeid()`` on uninitialised variables no longer
  produce a warning.
* The C++ ``typeid()`` function was allowed in C mode.
* The error position reported for errors found in f-strings was misleading.
* The new ``c_api_binop_methods`` directive was added for forward compatibility, but can
  only be set to True (the current default value).  It can be disabled in Cython 3.0.

(adam)