Ensure close is called when Container is deleted

Currently, close() is only called when using a context manager. This can
demonstrably lead to hangs when using AUTO threading mode as
shown in the cookbook example.

As @jlaine discovered in PyAV-Org#909, calling `close()` on the Container
seems to resolve the issue, likely by making sure the threads have
been joined by the time __dealloc__ runs.

av/container/core.pyx

@@ -244,6 +244,9 @@ cdef class Container(object):
             # Finish releasing the whole structure.
             lib.avformat_free_context(self.ptr)
 
+    def __del__(self):
+        self.close()
+
     def __enter__(self):
         return self