Revision 1889487

swig-py: Fix doubly destroying memory pool because weakref's callback is not
invoked when it is finalized by cyclic garbage collector.

See also https://bugs.python.org/issue40312 for weakref's callbacks during
garbage collection.

* subversion/bindings/swig/include/proxy_apr.swg
  (apr_pool_t.valid): Check whether parent pool is valid.

* subversion/bindings/swig/python/tests/pool.py
  (PoolTestCase): Add tests for pools referred from circular reference.