Fix asyncpg with Py_DEBUG mode

[shadchin]

If Py_DEBUG enabled, then newly allocated memory is filled with the byte 0xCD (CLEANBYTE) https://docs.python.org/3/c-api/memory.html#c.PyMem_SetupDebugHooks

This breaks checks for pointer == NULL and results in crash.

From documentation PyObject_GC_Track https://docs.python.org/3/c-api/gcsupport.html#c.PyObject_GC_Track

This should be called once all the fields followed by the tp_traverse handler become valid, usually near the end of the constructor.

[elprans]

Good catch. Thanks!