Add newsfragment for GC hooks

Author: oremanj

newsfragments/15.feature.rst

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+Add support for PEP 525-style finalization hooks via ``set_asyncgen_hooks()``
+and ``get_asyncgen_hooks()`` functions, which mimic the behavior of the ones
+in ``sys`` if we're running on a Python version that doesn't have them natively.
+``@async_generator`` generators behave the same way as native ones with respect
+to these hooks.